Semiconductor memory device capable of controlling potential level of power supply line and/or ground line

ABSTRACT

Level control signals are both set to H level, and potentials of power supply lines are both set to be lower than a power supply potential. In this manner, a gate leakage current during waiting and writing operation of a memory cell array can significantly be reduced. The level control signals are set to L level and H level respectively, and solely the potential of one of the power supply lines is set to be lower than the power supply potential. In this manner, power consumption during a reading operation of the memory cell array can be reduced.

RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.11/889,393, filed Aug. 13, 2007, which is a Divisional of U.S.application Ser. No. 11/086,345, filed Mar. 23, 2005, now U.S. Pat. No.7,286,391, which is a Divisional of U.S. application Ser. No.10/689,344, filed Oct. 21, 2003, now U.S. Pat. No. 6,903,962, claimingpriority of Japanese Application No. 2003-161115, filed Jun. 5, 2003,the entire contents of each of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory device, and moreparticularly to a semiconductor memory device capable of holding storagedata without a refresh operation.

2. Description of the Background Art

In a semiconductor memory device, in particular in an SRAM (StaticRandom Access Memory), power consumption can be lowered by controlling asource potential of a transistor constituting a memory cell so as tosuppress a leakage current that flows between the source and the drain,for example.

A conventional semiconductor memory device (a semiconductor device)disclosed in Japanese Patent Laying-Open No. 9-73784 reduces the leakagecurrent by maintaining a reading speed by setting the source potentialequal to a substrate potential during an operation, and by setting anabsolute potential of the source to be higher than the substratepotential during waiting. Though the conventional semiconductor memorydevice disclosed in Japanese Patent Laying-Open No. 9-73784 effectivelyreduces the leakage current during waiting, it does not reduce theleakage current during the operation. Therefore, lowering of powerconsumption during the operation is not expected.

Generally, power consumed during operation of the semiconductor memorydevice is the sum of power consumption by charging/discharging currentof a bit line or the like and power consumption by the leakage current.Though the charging/discharging current of the bit line or the like hasaccounted for a large part of the power consumption so far, powerconsumption by the leakage current during operation is not negligiblewhen the threshold value is set lower in accordance with the higherspeed of the semiconductor memory device.

A conventional semiconductor memory device (a semiconductor integratedcircuit) disclosed in Japanese Patent Laying-Open No. 2002-288984reduces the leakage current by setting the source potential of aselected memory cell row to be equal to the substrate potential and bysetting the absolute potential of the source of a non-selected memorycell row to be higher than the substrate potential during a readingoperation. The conventional semiconductor memory device disclosed inJapanese Patent Laying-Open No. 2002-288984 can suppress the leakagecurrent in memory cells other than the selected memory cell even duringthe operation. For example, in the case of a semiconductor memory deviceincluding a memory cell array of 512 rows and 512 columns, 512 memorycells, that are equivalent to one row, are selected, and hence anincrease of the overall leakage current is suppressed to 1/512.

As described above, the conventional semiconductor memory devicesdisclosed in the references above have lowered power consumption bycontrolling the source potential so as to suppress the leakage currentthat flows between the source and the drain. On the other hand, thesesemiconductor memory devices cannot reduce power consumption by thecharging/discharging current of the bit line or the like, which is onefactor of the power consumption during operation.

In addition, with regard to the conventional semiconductor memorydevices disclosed in the references above, only an example of asingle-port memory cell constituted of 6 transistors has been shown, andlower power consumption in an example of a multi-port memory cellincluding a bit line dedicated for reading has not been shown.

Moreover, with regard to the conventional semiconductor memory devicesdisclosed in the references above, the leakage current between thesource and the drain in a transistor that has turned off is solelyconsidered. Namely, an influence of the gate leakage current, which hasbecome apparent as a gate insulating film is made thinner, has not beenaddressed.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductor memorydevice capable of reducing power consumption caused by acharging/discharging current of a bit line or the like as well as causedby a gate leakage current of a memory cell in a non-selected column.

A semiconductor memory device according to one aspect of the presentinvention includes a plurality of memory cells arranged in matrix ofrows and columns, and a plurality of write word lines arrangedindividually for each of the plurality of memory cells. Each of theplurality of memory cells includes a data storage portion holding data,a data write portion writing data into the data storage portion, and adata read portion having a read bit line for reading data from the datastorage portion. The data storage portion has first and second invertercircuits connected in common to a power supply line arrangedcorresponding to respective columns of the plurality of memory cells.The semiconductor memory device further includes a power supply linelevel control circuit controlling a potential level of the power supplyline to a power supply potential or to a prescribed potential levellower than the power supply potential in response to a level controlsignal set for each column.

A semiconductor memory device according to another aspect of the presentinvention includes a plurality of memory cells arranged in matrix ofrows and columns, and a plurality of write word lines arrangedindividually for each of the plurality of memory cells. Each of theplurality of memory cells includes a data storage portion holding data,a data write portion writing data into the data storage portion, and adata read portion having a read bit line for reading data from the datastorage portion. The data storage portion has a first inverter circuitconnected to a first power supply line arranged corresponding torespective columns of the plurality of memory cells, and a secondinverter circuit connected to a second power supply line arrangedcorresponding to respective columns of the plurality of memory cells.The semiconductor memory device further includes a power supply linelevel control circuit controlling a potential level of the second powersupply line to a power supply potential or to a prescribed potentiallevel lower than the power supply potential for each column in responseto a level control signal set for each column.

A semiconductor memory device according to yet another aspect of thepresent invention includes a plurality of memory cells arranged inmatrix of rows and columns, and a plurality of write word lines arrangedindividually for each of the plurality of memory cells. Each of theplurality of memory cells includes a data storage portion holding data,a data write portion writing data into the data storage portion, and adata read portion having a read bit line for reading data from the datastorage portion. The data storage portion has a first inverter circuitoperating at a power supply potential or a prescribed potential levellower than the power supply potential in response to a level controlsignal set for each column, and a second inverter circuit operating atthe prescribed potential level.

A semiconductor memory device according to yet another aspect of thepresent invention includes a plurality of memory cells arranged inmatrix of rows and columns, and a plurality of write word lines arrangedindividually for each of the plurality of memory cells. Each of theplurality of memory cells includes a data storage portion holding data,a data write portion writing data into the data storage portion, and adata read portion having a read bit line for reading data from the datastorage portion. The data storage portion has a first inverter circuitoperating at a power supply potential or a prescribed potential levellower than the power supply potential in response to a level controlsignal set for each column and a second level control signal set foreach row, and a second inverter circuit operating at the prescribedpotential level.

A semiconductor memory device according to yet another aspect of thepresent invention includes a plurality of memory cells arranged inmatrix of rows and columns, and a plurality of write word lines arrangedindividually for each of the plurality of memory cells. Each of theplurality of memory cells includes a data storage portion holding data,a data write portion writing data into the data storage portion, and adata read portion having a read bit line for reading data from the datastorage portion. The data storage portion has first and second invertercircuits connected in common to a ground line arranged corresponding torespective columns of the plurality of memory cells. The semiconductormemory device further includes a ground line level control circuitcontrolling a potential level of the ground line to a ground potentialor to a prescribed potential level higher than the ground potential inresponse to a level control signal set for each column.

A semiconductor memory device according to yet another aspect of thepresent invention includes a plurality of memory cells arranged inmatrix of rows and columns, a plurality of word lines arranged for eachrow of the plurality of memory cells, and a plurality of bit line pairsarranged for each column of the plurality of memory cells. Each of theplurality of memory cells includes a data storage portion holding data,and a data write/read portion performing write/read of data to/from thedata storage portion. The data storage portion has first and secondinverter circuits connected in common to a power supply line arrangedcorresponding to respective columns of the plurality of memory cells.The semiconductor memory device further includes a power supply linelevel control circuit controlling a potential level of the power supplyline to a power supply potential or to a prescribed potential levellower than the power supply potential for each column in response to alevel control signal set for each column.

A semiconductor memory device according to yet another aspect of thepresent invention includes a plurality of memory cells arranged inmatrix of rows and columns, a plurality of word lines arranged for eachrow of the plurality of memory cells, and a plurality of bit line pairsarranged for each column of the plurality of memory cells. Each of theplurality of memory cells includes a data storage portion holding data,and a data write/read portion performing write/read of data to/from thedata storage portion. The data storage portion has first and secondinverter circuits connected in common to a ground line arrangedcorresponding to respective columns of the plurality of memory cells.The semiconductor memory device further includes a ground line levelcontrol circuit controlling a potential level of the ground line to aground potential or to a prescribed potential level higher than theground potential for each column in response to a level control signalset for each column.

Therefore, according to the present invention, power consumption by acharging/discharging current of a bit line or the like as well as by agate leakage current of a memory cell in a non-selected column can bereduced.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing a general configuration of asemiconductor memory device 100 in embodiments of the present invention.

FIG. 2 is a circuit diagram showing a circuit configuration of a memorycell array 110A and its periphery in Embodiment 1 of the presentinvention.

FIG. 3 is a circuit diagram showing a specific circuit configuration ofa memory cell 1A in Embodiment 1 of the present invention.

FIG. 4 is a timing chart for illustrating a writing operation at a writeport 2000A of memory cell 1A in Embodiment 1 of the present invention.

FIG. 5 is a timing chart for illustrating a reading operation at a readport 3000A of memory cell 1A in Embodiment 1 of the present invention.

FIG. 6 is a graph showing one example of a relation between a gateleakage current and a gate voltage in an MOS transistor.

FIG. 7 shows potential change of a read word line RWL[0] and read bitlines RBL[0], RBL[1] in memory cell array 110A in Embodiment 1.

FIG. 8 shows potential change of power supply lines VM[0], VM[1] inmemory cell array 110A in Embodiment 1.

FIG. 9 is a circuit diagram showing a circuit configuration of a memorycell array 110B and its periphery in Embodiment 2 of the presentinvention.

FIG. 10 is a circuit diagram showing a specific circuit configuration ofa memory cell 1B in Embodiment 2 of the present invention.

FIG. 11 is a circuit diagram showing a circuit configuration of a memorycell array 110C in Embodiment 3 of the present invention.

FIG. 12 is a circuit diagram showing a specific circuit configuration ofa memory cell 1C in Embodiment 3 of the present invention.

FIG. 13 is a circuit diagram showing a circuit configuration of a memorycell array 110D in Embodiment 4 of the present invention.

FIG. 14 is a circuit diagram showing a specific circuit configuration ofa memory cell 1D in Embodiment 4 of the present invention.

FIG. 15 is a circuit diagram showing a circuit configuration of a memorycell array 110E and its periphery in Embodiment 5 of the presentinvention.

FIG. 16 is a circuit diagram showing a circuit configuration of a groundline level control circuit 30E-1, which is one example of a ground linelevel control circuit 30E.

FIG. 17 is a circuit diagram showing a circuit configuration of a groundline level control circuit 30E-2, which is another example of groundline level control circuit 30E.

FIG. 18 is a circuit diagram showing a specific circuit configuration ofa memory cell 1E in Embodiment 5 of the present invention.

FIG. 19 shows potential change of read word line RWL[0] and read bitlines RBL[0], RBL[1] in memory cell array 110E in Embodiment 5.

FIG. 20 shows potential change of ground lines GM[0], GM[1] in memorycell array 110E in Embodiment 5.

FIG. 21 is a circuit diagram showing a circuit configuration of a memorycell array 110F and its periphery in Embodiment 6 of the presentinvention.

FIG. 22 is a circuit diagram showing a specific circuit configuration ofa memory cell 1F in Embodiment 6 of the present invention.

FIG. 23 shows potential change of ground lines GG[0], GG[1] in memorycell array 110F in Embodiment 6.

FIG. 24 is a circuit diagram showing a circuit configuration of a memorycell array 110G and its periphery in Embodiment 7 of the presentinvention.

FIG. 25 is a circuit diagram showing a specific circuit configuration ofa memory cell 1G in Embodiment 7 of the present invention.

FIG. 26 shows potential change of a word line WL[0], a bit line pairBL[0], /BL[0], and a bit line pair BL[1], /BL[1] in memory cell array110G in Embodiment 7.

FIG. 27 shows potential change of power supply lines VM[0], VM[1] andground lines GG[0], GG[1] in memory cell array 110G in Embodiment 7.

FIG. 28 is a circuit diagram showing a circuit configuration of a powersupply line level control circuit 20 in Embodiment 8 of the presentinvention.

FIG. 29 is a circuit diagram showing a specific circuit configuration ofa power supply line level switching circuit 200 in Embodiment 8 of thepresent invention.

FIG. 30 illustrates an operation of power supply line level switchingcircuit 200 in Embodiment 8 of the present invention.

FIG. 31 is a circuit diagram showing a circuit configuration of a groundline level control circuit 30 in Embodiment 9 of the present invention.

FIG. 32 is a circuit diagram showing a specific circuit configuration ofa ground line level switching circuit 300 in Embodiment 9 of the presentinvention.

FIG. 33 illustrates an operation of ground line level switching circuit300 in Embodiment 9 of the present invention.

FIG. 34 is a circuit diagram showing a circuit configuration of asetting signal control circuit 500 in Embodiment 10 of the presentinvention.

FIGS. 35A to 35D are operational waveform diagrams for illustrating anoperation of setting signal control circuit 500 respectively inEmbodiment 10 of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the present invention will be describedin detail with reference to the figures. It is noted that the samereference characters refer to the same or corresponding components inthe figures, and description thereof will not be repeated.

FIG. 1 is a schematic block diagram showing a general configuration of asemiconductor memory device 100 in embodiments of the present invention.

Semiconductor memory device 100 is a static memory device (SRAM, forexample) capable of holding storage data without a refresh operation.

Referring to FIG. 1, semiconductor memory device 100 includes a rowaddress terminal 102 receiving row address signals RA0 to RAi (i:natural number), a column address terminal 103 receiving column addresssignals CA0 to CAj (j: natural number), a control signal terminal 104receiving control signals such as a read/write control signal /W, a chipselect signal /CS, an output enable signal /OE, and the like, a datainput terminal 105 receiving input data D, and a data output terminal106 providing output data Q. Note that a symbol “/” preceding a signalrepresents inversion of that signal.

Semiconductor memory device 100 further includes a memory cell array 110having a plurality of memory cells arranged in matrix of rows andcolumns, a row decoder 120 decoding row address signals RA0 to RAi andselecting a memory cell row, a column decoder 130 decoding columnaddress signals CA0 to CAj and selecting a memory cell column, a controlcircuit 140 controlling an internal operation of semiconductor memorydevice 100 in response to a control signal, and a data input/outputcircuit 150 receiving/providing data between a data I/O line 160 anddata input/output terminals 105, 106.

Column decoder 130 includes a column select circuit coupling one of bitlines BLs provided corresponding to memory cell columns respectively todata I/O line 160, or the like. Data input/output circuit 150 includes awrite driver for writing input data D into memory cell array 110 viadata I/O line 160, an amplifier circuit for amplifying read datatransmitted to data I/O line 160, or the like. In the following,detailed description of memory cell array 110 and its peripheralcircuit, or memory cell array 110 will be provided for each Embodiment 1to 7.

EMBODIMENT 1

FIG. 2 is a circuit diagram showing a circuit configuration of a memorycell array 110A and its periphery in Embodiment 1 of the presentinvention. Memory cell array 110A of Embodiment 1 shown in FIG. 2 has amemory cell array configuration of two rows and two columns, forexample.

Referring to FIG. 2, memory cell array 110A in Embodiment 1 includesmemory cells 1A-0, 1A-1, 1A-2, 1A-3 arranged in matrix of rows andcolumns, write word lines WWLA[0], WWLA[1], WWLB[0], WWLB[1] and readword lines RWL[0], RWL[1] arranged in a direction of row, and write bitlines WBL[0], WBL[1] and read bit lines RBL[0], RBL[1] arranged in adirection of column.

Write word lines WWLA[0], WWLB[0] are connected to memory cells 1A-0,1A-1 respectively, whereas write word lines WWLA[1], WWLB[1] areconnected to memory cells 1A-2, 1A-3 respectively. In this manner, inmemory cell array 110A of Embodiment 1, a separate write word line isallocated to each memory cell, even if memory cells are located in thesame row. Accordingly, data can be written into a memory cell in aspecific column in a memory cell consisting of a plurality of columns.

Read word line RWL[0] is connected in common to memory cells 1A-0, 1A-1,while read word line RWL[1] is connected in common to memory cells 1A-2,1A-3. Write bit line WBL[0] is connected in common to memory cells 1A-0,1A-2, while write bit line WBL[1] is connected in common to memory cells1A-1, 1A-3. Read bit line RBL[0] is connected in common to memory cells1A-0, 1A-2, while read bit line RBL[1] is connected in common to memorycells 1A-1, 1A-3.

Memory cell array 110A in Embodiment 1 is connected to a power supplyline level control circuit 20A via power supply lines VM[0], VM[1].Power supply line VM[0] is connected in common to memory cells 1A-0,1A-2, whereas power supply line VM[1] is connected in common to memorycells 1A-1, 1A-3.

Power supply line level control circuit 20A includes P-channel MOStransistors 21A, 22A connected to power supply line VM[0] and P-channelMOS transistors 23A, 24A connected to power supply line VM[1]. P-channelMOS transistor 21A is diode-connected to power supply line VM[0], whileP-channel MOS transistor 23A is diode-connected to power supply lineVM[1].

P-channel MOS transistors 21A to 24A have the sources provided with thepower supply potential. P-channel MOS transistors 22A, 24A receive levelcontrol signals /CS[0], /CS[1] at their gates respectively.

Power supply line level control circuit 20A sets the potential level ofpower supply line VM[0] to a power supply potential VDD when levelcontrol signal /CS[0] is at L level, and sets the potential level ofpower supply line VM[0] to VDD−Vtp (Vtp represents a voltage between thegate and the source of a P-channel MOS transistor) when level controlsignal /CS[0] is at H level. Similarly, power supply line level controlcircuit 20A sets the potential level of power supply line VM[1] to powersupply potential VDD when level control signal /CS[1] is at L level, andsets the potential level of power supply line VM[1] to VDD−Vtp whenlevel control signal /CS[1] is at H level.

A specific circuit configuration of a memory cell 1A representing memorycells 1A-0, 1A-1, 1A-2, 1A-3 will now be described. Here, memory cell 1Ais assumed to include control lines such as a write word line or a readbit line.

FIG. 3 is a circuit diagram showing a specific circuit configuration ofmemory cell 1A in Embodiment 1 of the present invention.

Memory cell 1A in Embodiment 1 shown in FIG. 3 has a 2-port memory cellconfiguration, which is one example of a multi-port memory cell. Memorycell 1A includes a data storage portion 1000A, a write port 2000A, and aread port 3000A.

Data storage portion 1000A includes inverters 2A, 3A. Inverter 2Aincludes a P-channel MOS transistor 11 connected between power supplyline VM and a storage node N1, and an N-channel MOS transistor 12connected between storage node N1 and a ground line GND. Inverter 3Aincludes a P-channel MOS transistor 13 connected between power supplyline VM and a storage node N2, and an N-channel MOS transistor 14connected between storage node N2 and ground line GND.

The gates of P-channel MOS transistor 11 and N-channel MOS transistor 12are both connected to storage node N2. The gates of P-channel MOStransistor 13 and N-channel MOS transistor 14 are both connected tostorage node N1.

Write port 2000A includes an N-channel MOS transistor 4, a write wordline WWL, and a write bit line WBL. N-channel MOS transistor 4 has itssource connected to storage node N1, has its gate connected to writeword line WWL, and has its drain connected to write bit line WBL.

Read port 3000A includes N-channel MOS transistors 5, 6, a read wordline RWL, and a read bit line RBL. N-channel MOS transistor 5 has itssource connected to ground line GND, has its gate connected to storagenode N2, and has its drain connected to the source of N-channel MOStransistor 6. N-channel MOS transistor 6 has its source connected to thedrain of N-channel MOS transistor 5, has its gate connected to read wordline RWL, and has its drain connected to read bit line RBL.

As described above, memory cell 1A in Embodiment 1 has a 2-port memorycell configuration including write port 2000A and read port 3000A.Therefore, read bit line RBL is not electrically connected to storagenodes N1, N2. Accordingly, destruction of the storage data during thereading operation can be prevented, and stable reading operation can beachieved.

A detailed circuit operation of memory cell 1A will be described indetail with reference to FIGS. 4 and 5.

FIG. 4 is a timing chart for illustrating a writing operation at writeport 2000A of memory cell 1A in Embodiment 1 of the present invention.

Initially, before the writing operation, a not-shown drive circuitdrives write bit line WBL to H level (logic high) and L level (logiclow) respectively when H level and L level are written into storage nodeN1 respectively.

When write word line WWL rises from L level to H level and the writingoperation is started at time t1, N-channel MOS transistor 4 (accesstransistor) in FIG. 3 turns on, and write bit line WBL is electricallyconnected to storage node N1.

Here, as write bit line WBL is strongly driven, storage node N1 variesto a level of write bit line WBL regardless of a state of the held data.Storage node N2 varies to a level opposite to that of storage node N1.In FIG. 4, write bit line WBL is assumed to be driven to L level.Therefore, storage nodes N1, N2 attain L level and H level respectivelyat time t1.

When write word line WWL falls from H level to L level at time t2,N-channel MOS transistor 4 (access transistor) in FIG. 3 turns off, andwrite bit line WBL is electrically isolated from storage node N1.

In response to electrical isolation of write bit line WBL from storagenode N1, storage nodes N1, N2 are stabilized at a lead-in levelrespectively. As a result, storage nodes N1, N2 holds each data, and thewriting operation is completed.

FIG. 5 is a timing chart for illustrating a reading operation at readport 3000A of memory cell 1A in Embodiment 1 of the present invention.

Initially, before the reading operation, a not-shown precharge circuitprecharges in advance read bit line RBL to H level. In the following, anexample in which storage nodes N1, N2 are at L level and H levelrespectively will be described.

When read word line RWL rises from L level to H level and the readingoperation starts at time t1, N-channel MOS transistor 6 in FIG. 3 turnson, and read bit line RBL is electrically connected to ground line GNDbecause storage node N2 is also at H level. As a result, read bit lineRBL attains L level, leading to reading of L level, which is an invertedlevel of storage node N2.

When read word line RWL falls from H level to L level at time t2,N-channel MOS transistor 6 in FIG. 3 turns off, and read bit line RBL iselectrically isolated from ground line GND.

At time t3, read bit line RBL is again precharged to H level for a nextreading operation, and the reading operation is completed.

If storage nodes N1, N2 are at H level and L level respectively, readbit line RBL is not electrically connected to ground line GND even ifread word line RWL rises from L level to H level. This is becausestorage node N2 is at L level. Therefore, read bit line RBL maintains Hlevel, leading to reading of H level, which is an inverted level ofstorage node N2.

An operation of memory cell array 110A of which circuit configurationhas been described in conjunction with FIG. 2 will now be described indetail.

First, an operation of memory cell array 110A during waiting in whichneither writing nor reading is performed will be described.

Referring to FIG. 2, in memory cell array 110A during waiting, writeword lines WWLA[0], WWLA[1], WWLB[0], WWLB[1] and read word linesRWL[0], RWL[L] are all set to L level. In other words, none of memorycells 1A-0, 1A-1, 1A-2, 1A-3 is selected during waiting.

During waiting, level control signals /CS[0], /CS[1] are both set to Hlevel.

By setting both of level control signals /CS[0], /CS[1] to H level,P-channel MOS transistors 22A, 24A both enter off state. In response tothis, the gate leakage current flows in an MOS transistor within memorycells 1A-0, 1A-1, 1A-2, 1A-3, and power supply lines VM[0], VM[1] areboth stabilized at a potential as low as VDD−Vtp (Vtp is a voltagebetween the gate and the source of a P-channel MOS transistor).

A relation between the gate leakage current as above and the gatevoltage (generic name of the voltage between the gate and the source andthe voltage between the gate and the drain) will be described withreference to FIG. 6.

FIG. 6 is a graph showing one example of a relation between the gateleakage current and the gate voltage in an MOS transistor.

The graph in FIG. 6 shows an example in which a gate oxide film of theMOS transistor has a thickness of 20 Å, wherein the abscissa representsthe gate voltage (unit: V), and the ordinate represents the gate leakagecurrent (unit: A/μm²) that flows per unit gate area. Note that theordinate is represented by logarithmic scale.

As shown in FIG. 6, though the gate leakage current attains 10⁻¹¹ A/μm²when the gate voltage is set to 1.0V, the gate leakage current isreduced by one digit to 10⁻¹² A/μm² when the gate voltage is lowered to0.5V. In this manner, since the gate leakage current exponentiallyvaries with respect to the gate voltage, slight lowering of the gatevoltage will result in significant reduction in the gate leakagecurrent.

Referring to FIG. 3, if storage node N2 of memory cell 1A is at H levelfor example, the gate leakage current flows from each gate terminal ofN-channel MOS transistors 5, 12 to ground line GND. If the potential ofpower supply line VM should fall from 1.0V to 0.5V, the gate leakagecurrent in N-channel MOS transistors 5, 12 is reduced by one digit.

Therefore, referring to FIG. 2, by setting level control signals /CS[0],/CS[1] to H level so as to lower the potential of power supply linesVM[0], VM[1] to VDD−Vtp, the gate leakage current in memory cell array110A during waiting can significantly be reduced. Thus, powerconsumption of memory cell array 110A during waiting can significantlybe reduced.

A writing operation of memory cell array 110A will now be described withreference to FIG. 2.

When data is written into memory cell 1A-0 for example, write bit lineWBL[0] is selected by a column address signal, and write bit line WBL[0]is driven to a desired value. In succession, write word line WWLA[0] isselected by a column address signal and a row address signal and set toH level, whereby desired data is written into memory cell 1A-0.

When data is written into memory cell 1A-1, write bit line WBL[1] isselected by a column address signal, and write bit line WBL[1] is drivento a desired value. In succession, write word line WWLB[0] is selectedby a column address signal and a row address signal and set to H level,whereby desired data is written into memory cell 1A-1.

In this writing operation, level control signals /CS[0], /CS[1] are bothset to H level.

By setting both of level control signals /CS[0], /CS[1] to H level, thegate leakage current flows in the MOS transistor within memory cells1A-0, 1A-1, 1A-2, 1A-3, and power supply lines VM[0], VM[1] are bothstabilized at a potential as low as VDD−Vtp (Vtp is a voltage betweenthe gate and the source of a P-channel MOS transistor).

Therefore, by setting level control signals /CS[0], /CS[1] to H level soas to lower the potential of power supply lines VM[0], VM[1] to VDD−Vtp,the gate leakage current during writing operation of memory cell array110A can significantly be reduced. Thus, power consumption duringwriting operation of memory cell array 110A can significantly bereduced.

A reading operation of memory cell array 110A will now be described. Inthe following, an example in which data in memory cell 1A-0 is read willbe described with reference to FIGS. 7 and 8.

FIG. 7 shows potential change of read word line RWL[0] and read bitlines RBL[0], RBL[1] in memory cell array 110A in Embodiment 1.

When data in memory cell 1A-0 is read, read word line RWL[0] is selectedby a row address signal and set to H level (power supply potential VDD)as shown in FIG. 7. Desired data is thus read to read bit line RBL[0].Power supply potential VDD is set to 1.0V, for example.

Note that read word line RWL[0] is also connected to memory cell 1A-1,which is a memory cell in a non-read column and in the same row, asshown in FIG. 2. Therefore, when the data in memory cell 1A-0 is read toread bit line RBL[0], the data in memory cell 1A-1 is simultaneouslyread to read bit line RBL[1].

In memory cell array 110A in Embodiment 1, each data in memory cells1A-0, 1A-1 that has been read simultaneously is input to a not-shownselector circuit. By selecting the data of one read bit line designatedby a column address signal, desired data is read.

Meanwhile, as described with reference to FIG. 5, the reading operationof memory cell 1A is performed after the read bit line is precharged inadvance to H level.

Accordingly, when the data at H level is read to read bit line RBL[0](when storage node N2 in FIG. 3 is at L level), the level of read bitline RBL[0] is not varied by the reading operation. On the other hand,when the data at L level is read to read bit line RBL[0] (when storagenode N2 in FIG. 3 is at H level), the level of read bit line RBL[0]varies from H level to L level by the reading operation.

Here, desirably, the potential level of read bit line RBL[0] of a dataread column varies rapidly in order to attain higher speed in thereading operation. On the other hand, desirably, the potential level ofread bit line RBL[1] of a data non-read column does not vary, in orderto suppress the charging/discharging current to reduce powerconsumption.

In memory cell array 110A in Embodiment 1, during the reading operation,level control signal /CS[0] controlling the potential level of powersupply line VM[0] of the data read column is set to L level, and levelcontrol signal /CS[1] controlling the potential level of power supplyline VM[1] of the data non-read column is set to H level, respectively.

FIG. 8 shows potential change of power supply lines VM[0], VM[1] inmemory cell array 110A in Embodiment 1.

By setting level control signals /CS[0], /CS[1] to L level and H levelrespectively, in the reading operation, the potential level at powersupply line VM[0] in the data read column (selected column) is set topower supply potential VDD, and the potential level at power supply lineVM[1] in the data non-read column (non-selected column) is set toVDD−Vtp (Vtp is the voltage between the gate and the source of aP-channel MOS transistor), as shown in FIG. 8.

Therefore, the potential level of storage node N2 in FIG. 3 in selectedmemory cell 1A-0 is set to power supply potential VDD, and a voltage ofpower supply potential VDD is applied between the gate and the source ofN-channel MOS transistor 5 for reading in FIG. 3. Generally, the higherthe voltage between the gate and the source is, the higher thedrivability of an MOS transistor will be. Therefore, data of read bitline RBL[0] in the data read column is rapidly pulled.

Consequently, referring to FIG. 7, the potential level of read bit lineRBL[0] in the data read column (selected column) is loweredsignificantly during the reading operation.

On the other hand, the potential level of storage node N2 in FIG. 3 innon-selected memory cell 1A-1 is set to VDD−Vtp, and a voltage ofVDD−Vtp is applied between the gate and the source of N-channel MOStransistor 5 for reading in FIG. 3. As non-selected memory cell 1A-1 hasthe voltage between the gate and the source of N-channel MOS transistor5 for reading lower than that of selected memory cell 1A-0, the data ofread bit line RBL[1] in the data non-read column is pulled slowly.

Consequently, referring to FIG. 7, the potential level of read bit lineRBL[1] in the data non-read column (non-selected column) is not loweredsignificantly during the reading operation.

When the potential change of read bit line RBL[0] is transmitted to dataoutput and the reading operation is completed, the potential level ofread bit lines RBL[0], RBL[1] returns to H level by precharging, asshown in FIG. 7.

Here, read bit line RBL[1] in the data non-read column of whichpotential level is not lowered significantly during the readingoperation can return to H level with a small amount of charging currentduring a precharge operation.

In this manner, level control signals /CS[0], /CS[1] are set to L leveland H level respectively, and the potential of power supply lines VM[0],VM[1] is set to VDD and VDD−Vtp respectively. Thus, power consumptionduring the reading operation of memory cell array 110A can be reduced.

When the reading operation is completed, level control signal /CS[0] isreturned to H level in order to reduce power consumption by the gateleakage current, and P-channel MOS transistor 22A in FIG. 2 that hasdriven the potential level of power supply line VM[0] to power supplypotential VDD is turned off. Accordingly, the potential level of powersupply line VM[0] is gradually lowered to the level of VDD−Vtp andstabilized thereat, as shown in FIG. 8.

As described above, according to Embodiment 1, separate write word lineis allocated to respective memory cell even if memory cells are locatedin the same row, and the potential of the power supply line iscontrolled in response to the level control signal. In this manner,power consumption by the charging/discharging current of the bit line orthe like as well as by the gate leakage current of the memory cell inthe non-selected column can be reduced.

EMBODIMENT 2

FIG. 9 is a circuit diagram showing a circuit configuration of a memorycell array 110B and its periphery in Embodiment 2 of the presentinvention.

Referring to FIG. 9, memory cell array 110B in Embodiment 2 includesmemory cells 1B-0, 1B-1, 1B-2, 1B-3 arranged in matrix of rows andcolumns, write word lines WWLA[0], WWLA[1], WWLB[0], WWLB[L] and readword lines RWL[0], RWL[L] arranged in a direction of row, and write bitlines WBL[0], WBL[L] and read bit lines RBL[0], RBL[1] arranged in adirection of column.

As a connection relation of the write word line, the read word line, thewrite bit line, and the read bit line to each memory cell is similar tothat in memory cell array 110A in Embodiment 1, description thereof willnot be repeated.

Memory cell array 110B in Embodiment 2 is connected to a power supplyline level control circuit 20B via power supply lines VM1[0], VM2[0],VM1[1], VM2[1]. Each of power supply lines VM1[0], VM2[0] is connectedin common to memory cells 1B-0, 1B-2, while each of power supply linesVM[1], VM2[1] is connected in common to memory cells 1B-1, 1B-3.

Power supply line level control circuit 20B includes a P-channel MOStransistor 21B connected to power supply line VM1[0], P-channel MOStransistors 22B, 23B connected to power supply line VM2[0], a P-channelMOS transistor 24B connected to power supply line VM1[1], and P-channelMOS transistors 25B, 26B connected to power supply line VM2[1].

P-channel MOS transistor 21B is diode-connected to power supply lineVM1[0]. P-channel MOS transistor 22B is diode-connected to power supplyline VM2[0]. P-channel MOS transistor 24B is diode-connected to powersupply line VM1[1]. P-channel MOS transistor 25B is diode-connected topower supply line VM2[1].

P-channel MOS transistors 21B to 26B have the sources provided withpower supply potential VDD. P-channel MOS transistors 23B, 26B receivelevel control signals /CS[0], /CS[1] at their gates respectively.

Power supply line level control circuit 20B sets the potential level ofpower supply line VM2[0] to power supply potential VDD when levelcontrol signal /CS[0] is at L level, and sets the potential level ofpower supply line VM2[0] to VDD−Vtp (Vtp represents a voltage betweenthe gate and the source of a P-channel MOS transistor) when levelcontrol signal /CS[0] is at H level. Similarly, power supply line levelcontrol circuit 20B sets the potential level of power supply line VM2[1]to power supply potential VDD when level control signal /CS[1] is at Llevel, and sets the potential level of power supply line VM2[1] toVDD−Vtp when level control signal /CS[1] is at H level. On the otherhand, each potential of power supply lines VM1[0], VM1[1] is fixed toVDD−Vtp.

A specific circuit configuration of a memory cell 1B representing memorycells 1B-0, 1B-1, 1B-2, 1B-3 will now be described. Here, memory cell 1Bis assumed to include control lines such as a write word line or a readbit line. With regard to the circuit operation of memory cell 1B andmemory cell array 110B in Embodiment 2, description of portions the sameas those in Embodiment 1 will not be repeated.

FIG. 10 is a circuit diagram showing a specific circuit configuration ofmemory cell 1B in Embodiment 2 of the present invention.

Referring to FIG. 10, memory cell 1B in Embodiment 2 includes a datastorage portion 1000B, a write port 2000B, and a read port 3000B. Datastorage portion 1000B includes inverters 2B, 3B. Here, as write port2000B and read port 3000B are identical to write port 2000A and readport 3000A in Embodiment 1 respectively, description thereof will not berepeated.

Data storage portion 1000B in Embodiment 2 is different from datastorage portion 1000A in Embodiment 1 only in that power supply line VM1having a fixed potential level is connected to inverter 2B, and powersupply line VM2 of which potential level can be controlled is connectedto inverter 3B. Even when the potential of power supply line VM1 isfixed, this merely means that the potential of storage node N1 is fixed.Therefore, the potential of storage node N2 affecting the reading speedcan be controlled in a manner the same as in Embodiment 1.

In this manner, whereas power supply line VM in Embodiment 1 isconnected in common to inverters 2B, 3B, power supply lines VM1, VM2 inEmbodiment 2 are individually connected to inverters 2B, 3Brespectively. Therefore, load capacitance onto power supply lines VM1,VM2 in Embodiment 2 is made smaller than that onto power supply line VMin Embodiment 1.

Thus, in memory cell array 110B in Embodiment 2, when the potentiallevel of the power supply line in the data read column (selected column)is raised from VDD−Vtp to VDD during reading operation as described inFIG. 8, required power consumption can be reduced, and the rising speedof the potential level is improved.

In addition, though H level at storage node N1 in Embodiment 1 has beenpower supply potential VDD, H level at storage node N1 in Embodiment 2is the potential level of VDD−Vtp even during the reading operation.

Therefore, memory cell array 110B in Embodiment 2 can reduce the gateleakage current caused by storage node N1 set to H level even during thereading operation.

As described above, according to Embodiment 2, the power supply line isdivided into two lines, and the potential level of only one of thoselines is allowed for control. In this manner, power consumption by thecharging/discharging current of the bit line or the like as well as bythe gate leakage current in the memory cell in the non-selected columncan be reduced.

EMBODIMENT 3

FIG. 11 is a circuit diagram showing a circuit configuration of a memorycell array 110C in Embodiment 3 of the present invention.

Referring to FIG. 11, memory cell array 110C in Embodiment 3 includesmemory cells 1C-0, 1C-1, 1C-2, 1C-3 arranged in matrix of rows andcolumns, write word lines WWLA[0], WWLA[1], WWLB[0], WWLB[1] and readword lines RWL[0], RWL[1] arranged in a direction of row, and write bitlines WBL[0], WBL[1] and read bit lines RBL[0], RBL[1] arranged in adirection of column.

As a connection relation of the write word line, the read word line, thewrite bit line, and the read bit line to each memory cell is similar tothat in memory cell array 110A in Embodiment 1, description thereof willnot be repeated.

In memory cell array 110C in Embodiment 3, level control signals /CS[0],/CS[1] are directly input to memory cells 1C-0, 1C-1, 1C-2, 1C-3. Levelcontrol signal /CS[0] is input in common to memory cells 1C-0, 1C-2,while level control signal /CS[1] is input in common to memory cells1C-1, 1C-3.

A specific circuit configuration of a memory cell 1C representing memorycells 1C-0, 1C-1, 1C-2, 1C-3 will now be described. Here, memory cell 1Cis assumed to include control lines such as a write word line or a readbit line. With regard to the circuit operation of memory cell 1C andmemory cell array 110C in Embodiment 3, description of portions the sameas those in Embodiment 1 will not be repeated.

FIG. 12 is a circuit diagram showing a specific circuit configuration ofmemory cell 1C in Embodiment 3 of the present invention.

Referring to FIG. 12, memory cell 1C in Embodiment 3 includes a datastorage portion 1000C, a write port 2000C, and a read port 3000C. Here,as write port 2000C and read port 3000C are identical to write port2000A and read port 3000A in Embodiment 1 respectively, descriptionthereof will not be repeated.

Data storage portion 1000C includes inverters 2C, 3C. Inverter 2Cincludes P-channel MOS transistor 11 connected between a node N3 andstorage node N1, N-channel MOS transistor 12 connected between storagenode N1 and ground node GND, and P-channel MOS transistor 15 connectedbetween power supply node VDD to which power supply potential VDD isprovided and node N3.

Inverter 3C includes P-channel MOS transistor 13 connected between nodeN3 and storage node N2, N-channel MOS transistor 14 connected betweenstorage node N2 and ground node GND, and P-channel MOS transistor 16diode-connected between power supply node VDD to which power supplypotential VDD is provided and node N3.

The gates of P-channel MOS transistor 11 and N-channel MOS transistor 12are both connected to storage node N2. The gates of P-channel MOStransistor 13 and N-channel MOS transistor 14 are both connected tostorage node N1. P-channel MOS transistor 15 receives a level controlsignal /CS linked to the column address signal, the write controlsignal, and the read control signal at its gate.

Referring to FIG. 12, when level control signal /CS is at L level,P-channel MOS transistor 15 turns on, and the potential of node N3attains power supply potential VDD. Accordingly, one potential higherthan another of storage nodes N1 and N2 attains power supply potentialVDD.

On the other hand, when level control signal /CS is at H level,P-channel MOS transistor 15 turns off. Therefore, the potential of nodeN3 gradually lowers, and is stabilized at a level lowered by Vtp, thatis, the voltage between the gate and the source of P-channel MOStransistor 16. In other words, as the potential of node N3 attainsVDD−Vtp, one potential higher than another of storage nodes N1 and N2attains VDD−Vtp.

In memory cell array 110C in Embodiment 3, level control signals /CS[0],/CS[1] are set to L level and H level respectively during the readingoperation of memory cell 1C-0, as in Embodiment 1.

In this manner, since the potential level of node N3 and storage node N2in memory cells 1C-0, 1C-2 in the data read column attains power supplypotential VDD, high-speed reading operation is achieved.

On the other hand, since the potential level of node N3 and storage nodeN2 in memory cells 1C-1, 1C-3 in the data non-read column attainsVDD−Vtp, power consumption by the gate leakage current can be reduced.

When storage node N2 in memory cell 1C-1 is at H level, data of read bitline RBL[L] is pulled slowly, as described in connection with FIG. 7 ofEmbodiment 1. Therefore, the potential level is not significantlylowered during the reading operation, and the charging/dischargingcurrent can be suppressed.

In addition, during the writing operation and waiting, as the potentiallevel of node N3 is lowered to VCC−Vtp in all memory cells 1C-0, 1C-1,1C-2, 1C-3, the gate leakage current can be reduced.

As described above, according to Embodiment 3, the level control signalis directly input to the memory cell so as to control the potentiallevel of the storage node. In this manner, power consumption by thecharging/discharging current of the bit line or the like as well as bythe gate leakage current of the memory cell in the non-selected columncan be reduced.

EMBODIMENT 4

FIG. 13 is a circuit diagram showing a circuit configuration of a memorycell array 110D in Embodiment 4 of the present invention.

Referring to FIG. 13, memory cell array 110D in Embodiment 4 includesmemory cells 1D-0, 1D-1, 1D-2, 1D-3 arranged in matrix of rows andcolumns, write word lines WWLA[0], WWLA[1], WWLB[0], WWLB[L] and readword lines RWL[0], RWL[L] arranged in a direction of row, and write bitlines WBL[0], WBL[1] and read bit lines RBL[0], RBL[1] arranged in adirection of column.

As a connection relation of the write word line, the read word line, thewrite bit line, and the read bit line to each memory cell is similar tothat in memory cell array 110A in Embodiment 1, description thereof willnot be repeated.

In memory cell array 110D in Embodiment 4, second level control signals/CR[0], /CR[1] in addition to level control signals /CS[0], /CS[1] areinput to memory cells 1D-0, 1D-1, 1D-2, 1D-3.

Level control signal /CS[0] is input in common to memory cells 1D-0,1D-2, while level control signal /CS[1] is input in common to memorycells 1D-1, 1D-3. Second level control signal /CR[0] is input in commonto memory cells 1D-0, 1D-1, while second level control signal /CR[1] isinput in common to memory cells 1D-2, 1D-3.

A specific circuit configuration of a memory cell 1D representing memorycells 1D-0, 1D-1, 1D-2, 1D-3 will now be described. Here, memory cell 1Dis assumed to include control lines such as a write word line or a readbit line. With regard to the circuit operation of memory cell 1D andmemory cell array 110D in Embodiment 4, description of portions the sameas those in Embodiment 1 will not be repeated.

FIG. 14 is a circuit diagram showing a specific circuit configuration ofmemory cell 1D in Embodiment 4 of the present invention.

Referring to FIG. 14, memory cell 1D in Embodiment 4 includes a datastorage portion 1000D, a write port 2000D, and a read port 3000D. Here,as write port 2000D and read port 3000D are identical to write port2000A and read port 3000A in Embodiment 1 respectively, descriptionthereof will not be repeated.

Data storage portion 1000D includes inverters 2D, 3D. Inverter 2Dincludes P-channel MOS transistor 11 connected between node N3 andstorage node N1, N-channel MOS transistor 12 connected between storagenode N1 and ground node GND, and P-channel MOS transistors 15, 17connected in series between power supply node VDD to which power supplypotential VDD is provided and node N3.

P-channel MOS transistor 15 receives level control signal /CS linked tothe column address signal, the write control signal, and the readcontrol signal at its gate. P-channel MOS transistor 17 receives levelcontrol signal /CS linked to the row address signal, the write controlsignal, and the read control signal at its gate.

Inverter 3D includes P-channel MOS transistor 13 connected between nodeN3 and storage node N2, N-channel MOS transistor 14 connected betweenstorage node N2 and ground node GND, and P-channel MOS transistor 16diode-connected between power supply node VDD to which power supplypotential VDD is provided and node N3.

The gates of P-channel MOS transistor 11 and N-channel MOS transistor 12are both connected to storage node N2. The gates of P-channel MOStransistor 13 and N-channel MOS transistor 14 are both connected tostorage node N1.

Referring to FIG. 14, when level control signal /CS and second levelcontrol signal /CR are both at L level, P-channel transistors 15, 17both turn on, and node N3 attains power supply potential VDD. Therefore,one potential higher than another of storage nodes N1 and N2 attainspower supply potential VDD.

On the other hand, when level control signal /CS or second level controlsignal /CR is at H level, either of P-channel transistors 15, 17 turnsoff. Specifically, when level control signal /CS is at H level,P-channel MOS transistor 15 turns off, and when second level controlsignal /CR is at H level, P-channel MOS transistor 17 turns off.

As such, the potential of node N3 gradually lowers, and is stabilized ata level lowered by Vtp, that is, the voltage between the gate and thesource of P-channel MOS transistor 16. In other words, as the potentialof node N3 attains VDD−Vtp, one potential higher than another of storagenodes N1 and N2 attains VDD−Vtp.

In memory cell array 110D in Embodiment 4, level control signal /CS[0]and second level control signal /CR[0] are set to L level in the readingoperation of memory cell 1D-0, so as to select a row and a column ofmemory cell 1D-0 from which data is read. In contrast, corresponding toa row and a column from which data is not read, level control signal/CS[1] and second level control signal /CR[1] are set to H level.

Thus, since the potential level of node N3 and storage node N2 in memorycell 1D-0 from which the data is read attains power supply potentialVDD, high-speed reading operation is achieved.

On the other hand, as the potential level of node N3 and storage node N2in memory cells 1D-1, 1D-2, 1D-3 from which the data is not read attainsVDD−Vtp, power consumption by the gate leakage current can be reduced.

The potential level of node N3 and storage node N2 in the memory cell inthe data read column has all attained power supply potential VDD inmemory cell array 100C in Embodiment 3. In memory cell array 110D ofEmbodiment 4, however, the potential level of node N3 and storage nodeN2 in the memory cell in a data non-read row attains VDD−Vtp even ifthat memory cell is in the data read column. Therefore, powerconsumption by the gate leakage current can further be reduced inEmbodiment 4, compared with Embodiment 3.

When storage node N2 in memory cell 1D-1 is at H level, the data of readbit line RBL[1] is slowly pulled, as described in connection with FIG. 7of Embodiment 1. Therefore, the potential level is not significantlylowered during the reading operation, and the charging/dischargingcurrent can be suppressed.

In addition, during the writing operation and waiting, as the potentiallevel of node N3 is lowered to VCC−Vtp in all memory cells 1D-0, 1D-1,1D-2, 1D-3, the gate leakage current can be reduced.

As described above, according to Embodiment 4, the level control signalcorresponding to a row and a column respectively is directly input tothe memory cell so as to control the potential level of the storagenode. In this manner, power consumption by the charging/dischargingcurrent of the bit line or the like as well as by the gate leakagecurrent of the memory cell in the non-selected column can be reduced.

EMBODIMENT 5

FIG. 15 is a circuit diagram showing a circuit configuration of a memorycell array 110E and its periphery in Embodiment 5 of the presentinvention.

Referring to FIG. 15, memory cell array 110E in Embodiment 5 includesmemory cells 1E-0, 1E-1, 1E-2, 1E-3 arranged in matrix of rows andcolumns, write word lines WWLA[0], WWLA[1], WWLB[0], WWLB[1] and readword lines RWL[0], RWL[1] arranged in a direction of row, and write bitlines WBL[0], WBL[1] and read bit lines RBL[0], RBL[1] arranged in adirection of column.

As a connection relation of the write word line, the read word line, thewrite bit line, and the read bit line to each memory cell is similar tothat in memory cell array 110A in Embodiment 1, description thereof willnot be repeated.

Memory cell array 110E in Embodiment 5 is connected to power supply linelevel control circuit 20A via power supply lines VM[0], VM[1], andconnected to a ground line level control circuit 30E via ground linesGM[0], GM[1]. Power supply line VM[0] and ground line GM[0] areconnected in common to memory cells 1E-0, 1E-2. Power supply line VM[1]and ground line GM[1] are connected in common to memory cells 1E-1,1E-3.

As the circuit configuration and operation of power supply line levelcontrol circuit 20A has been described in connection with FIG. 2 ofEmbodiment 1, description thereof will not be repeated. A circuitconfiguration and operation of ground line level control circuit 30Ewill be described with reference to FIGS. 16 and 17.

FIG. 16 is a circuit diagram showing a circuit configuration of a groundline level control circuit 30E-1, which is one example of ground linelevel control circuit 30E.

Referring to FIG. 16, ground line level control circuit 30E-1 includesan N-channel MOS transistor 31E connected to ground line GM[0], and anN-channel MOS transistor 32E connected to ground line GM[1].

N-channel MOS transistors 31E, 32E both have the sources provided withground potential GND. The gates of N-channel MOS transistors 31E, 32Ereceive level control signals CS[0], CS[1] respectively. Level controlsignals CS[0], CS[1] link to the column address signal and the readcontrol signal.

Ground line level control circuit 30E-1 sets the potential level ofground line GM[0] to ground potential GND when level control signalCS[0] is at H level, and sets the potential level of ground line GM[0]to floating when level control signal CS[0] is at L level. Similarly,ground line level control circuit 30E-1 sets the potential level ofground line GM[1] to ground potential GND when level control signalCS[1] is at H level, and sets the potential level of ground line GM[1]to floating when level control signal CS[1] is at L level.

FIG. 17 is a circuit diagram showing a circuit configuration of a groundline level control circuit 30E-2, which is another example of groundline level control circuit 30E.

Referring to FIG. 17, ground line level control circuit 30E-2 includes aP-channel MOS transistor 33E and an N-channel MOS transistor 34Econnected to ground line GM[0], and a P-channel MOS transistor 35E andan N-channel MOS transistor 36E connected to ground line GM[1].

P-channel MOS transistor 33E and N-channel MOS transistor 34E areconnected in series between power supply node VDD and ground node GND,and their gates both receive level control signal CS[0]. P-channel MOStransistor 35E and N-channel MOS transistor 36E are connected in seriesbetween power supply node VDD and ground node GND, and their gates bothreceive level control signal CS[1]. Level control signals CS[0], CS[1]link to the column address signal and the read control signal.

Ground line level control circuit 30E-2 sets the potential level ofground line GM[0] to ground potential GND when level control signalCS[0] is at H level, and sets the potential level of ground line GM[0]to power supply potential VDD when level control signal CS[0] is at Llevel. Similarly, ground line level control circuit 30E-2 sets thepotential level of ground line GM[1] to ground potential GND when levelcontrol signal CS[1] is at H level, and sets the potential level ofground line GM[1] to power supply potential VDD when level controlsignal CS[1] is at L level.

In this manner, ground line level control circuit 30E can adopt acircuit configuration of ground line level control circuit 30E-1, oralternatively, adopt a circuit configuration of ground line levelcontrol circuit 30E-2.

In other words, ground line level control circuit 30E sets the potentiallevel of ground line GM[0] to ground potential GND when level controlsignal CS[0] is at H level, and sets the potential level of ground lineGM[0] to power supply potential VDD or floating when level controlsignal CS[0] is at L level. Similarly, ground line level control circuit30E sets the potential level of ground line GM[1] to ground potentialGND when level control signal CS[1] is at H level, and sets thepotential level of ground line GM[1] to power supply potential VDD orfloating when level control signal CS[1] is at L level.

A specific circuit configuration of a memory cell 1E representing memorycells 1E-0, 1E-1, 1E-2, 1E-3 will now be described. Here, memory cell 1Eis assumed to include control lines such as a write word line or a readbit line.

FIG. 18 is a circuit diagram showing a specific circuit configuration ofmemory cell 1E in Embodiment 5 of the present invention.

Referring to FIG. 18, memory cell 1E in Embodiment 5 includes a datastorage portion 1000E, a write port 2000E, and a read port 3000E. Here,as data storage portion 1000E and write port 2000E are identical to datastorage portion 1000A and write port 2000A in Embodiment 1 respectively,description thereof will not be repeated.

Read port 3000E in Embodiment 5 is different from read port 3000A inEmbodiment 1 only in that ground line GM of which potential level can becontrolled is connected to the source of N-channel MOS transistor 5.

An operation of memory cell array 110E including memory cells 1E-0,1E-1, 1E-2, 1E-3 with the memory cell configuration as above will now bedescribed. Note that description of portions the same as those inEmbodiment 1 will not be repeated.

Referring to FIGS. 16 and 17, in memory cell array 110E in Embodiment 5,level control signals CS[0], CS[1] controlling the potential level ofground lines GM[0], GM[1] are both set to L level during non-readingoperation, that is, during waiting or the writing operation.

Accordingly, the potential level of ground lines GM[0], GM[1] are bothset to power supply potential VDD or floating. Referring to FIG. 18, thepotential level of ground lines GM[0], GM[1] attain the potential levelthe same as that of read bit lines RBL[0], RBL[L] that are precharged inadvance to power supply potential VDD before the reading operation,considering the voltage between the gate and the drain of N-channel MOStransistor 5.

A reading operation of memory cell array 110E will now be described. Inthe following, an example in which data in memory cell 1E-0 is read willbe described with reference to FIGS. 19 and 20.

FIG. 19 shows potential change of read word line RWL[0] and read bitlines RBL[0], RBL[1] in memory cell array 110E in Embodiment 5.

When the data in memory cell 1E-0 is read, read word line RWL[0] isselected by the row address signal and set to H level (power supplypotential VDD) as shown in FIG. 19. Desired data is thus read to readbit line RBL[0]. Power supply potential VDD is set to 1.0V, for example.

In memory cell array 110E in Embodiment 5, during the reading operation,level control signal CS[0] controlling the potential level of groundline GM[0] in the data read column is set to H level, and level controlsignal CS[1] controlling the potential level of ground line GM[1] in thedata non-read column is set to L level. In this manner, the potentiallevel of ground line GM[0] is set to ground potential GND, and thepotential level of ground line GM[1] is set to power supply potentialVDD or floating.

FIG. 20 shows potential change of ground lines GM[0], GM[1] in memorycell array 110E in Embodiment 5.

During the reading operation, level control signals CS[0], CS[1] are setto H level and L level respectively. Then, as shown in FIG. 20, thepotential level of ground line GM[0] in the data read column (selectedcolumn) is set to ground potential GND, and the potential level ofground line GM[1] in the data non-read column (non-selected column) isset to power supply potential VDD or floating.

Read bit line RBL[0] of the data read column (selected column) iselectrically connected to ground line GM, when the potential level ofstorage node N2 in FIG. 18 in selected memory cell 1E-0 is at H level.As a result, the potential level of read bit line RBL[0] graduallylowers toward ground potential GND, as shown in FIG. 19. Therefore, dataof L level is read to read bit line RBL[0].

On the other hand, read bit line RBL[1] of the data non-read column(non-selected column) maintains the potential level of power supplypotential VDD regardless of the potential level of storage node N2 inFIG. 18 in the non-selected memory cell (memory cell 1E-1, for example)as shown in FIG. 19, because read bit line RBL[1] and ground line GM[1]are both at H level.

In this manner, in memory cell array 110E of Embodiment 5, the potentiallevel of read bit line RBL[1] in the data non-read column (non-selectedcolumn) does not vary during the reading operation. Therefore, thecharging/discharging current in the data non-read column is completelyeliminated, and power consumption can be reduced.

As described above, according to Embodiment 5, the potential of theground line is controlled in response to the level control signal. Thus,power consumption by the charging/discharging current of the bit line orthe like as well as by the gate leakage current of the memory cell inthe non-selected column can be reduced.

It is to be noted that control of the potential of the ground line inresponse to the level control signal as in Embodiment 5 can also beadapted to Embodiments 1 to 4.

EMBODIMENT 6

FIG. 21 is a circuit diagram showing a circuit configuration of a memorycell array 110F and its periphery in Embodiment 6 of the presentinvention.

Referring to FIG. 21, memory cell array 110F in Embodiment 6 includesmemory cells 1F-0, 1F-1, 1F-2, 1F-3 arranged in matrix of rows andcolumns, write word lines WWLA[0], WWLA[1], WWLB[0], WWLB[1] and readword lines RWL[0], RWL[1] arranged in a direction of row, and write bitlines WBL[0], WBL[1] and read bit lines RBL[0], RBL[1] arranged in adirection of column.

As a connection relation of the write word line, the read word line, thewrite bit line, and the read bit line to each memory cell is similar tothat in memory cell array 110A in Embodiment 1, description thereof willnot be repeated.

Memory cell array 110F in Embodiment 6 is connected to ground line levelcontrol circuit 30E via ground lines GM[0], GM[1], and connected to aground line level control circuit 30F via ground lines GG[0], GG[1].Ground lines GM[0], GG[0] are connected in common to memory cells 1F-0,1F-2, while ground lines GM[1], GG[1] are connected in common to memorycells 1F-1, 1F-3.

As the circuit configuration and operation of ground line level controlcircuit 30E has been described in connection with FIGS. 16 and 17 inEmbodiment 5, description thereof will not be repeated.

Ground line level control circuit 30F includes N-channel MOS transistors31F, 32F connected to ground line GG[0], and N-channel MOS transistors33F, 34F connected to ground line GG[1]. N-channel MOS transistor 32F isdiode-connected to ground line GG[0], while N-channel MOS transistor 34Fis diode-connected to ground line GG[1].

N-channel MOS transistors 31F to 34F all have the sources provided withground potential GND. The gates of N-channel MOS transistors 31F, 33Freceive level control signals CS[0], CS[1] respectively. Level controlsignals CS[0], CS[1] link to the column address signal and the readcontrol signal.

Ground line level control circuit 30F sets the potential level of groundline GG[0] to ground potential GND when level control signal CS[0] is atH level, and sets the potential level of ground line GM[0] to Vtn (Vtnis a voltage between the gate and the source of an N-channel MOStransistor) when level control signal CS[0] is at L level. Similarly,ground line level control circuit 30F sets the potential level of groundline GG[1] to ground potential GND when level control signal CS[1] is atH level, and sets the potential level of ground line GM[1] to Vtn whenlevel control signal CS[1] is at L level.

A specific circuit configuration of a memory cell 1F representing memorycells 1F-0, 1F-1, 1F-2, 1F-3 will now be described. Here, memory cell 1Fis assumed to include control lines such as a write word line or a readbit line.

FIG. 22 is a circuit diagram showing a specific circuit configuration ofmemory cell 1F in Embodiment 6 of the present invention.

Referring to FIG. 22, memory cell 1F in Embodiment 6 includes a datastorage portion 1000F, a write port 2000F, and a read port 3000F. Datastorage portion 1000F includes inverters 2F, 3F. Here, as write port2000F is identical to write port 2000A in Embodiment 1, descriptionthereof will not be repeated. As read port 3000F is identical to readport 3000E in Embodiment 5, description thereof will not be repeated.

Data storage portion 1000F in Embodiment 6 is different from datastorage portion 1000A in Embodiment 1 only in that the potential levelof the power supply line is fixed to power supply potential VDD, andground line GG of which potential level can be controlled is connectedto inverters 2F, 3F.

An operation of memory cell array 110F including memory cells 1F-0,1F-1, 1F-2, 1F-3 with the memory cell configuration as above will now bedescribed. Note that description of portions the same as those inEmbodiment 1 will not be repeated. In addition, as control of thepotential level of ground lines GM[0], GM[1] by ground line levelcontrol circuit 30E has been described in Embodiment 5, descriptionthereof will not be repeated.

Referring to FIG. 21, in memory cell array 110F in Embodiment 6, levelcontrol signal CS[0], CS[1] controlling the potential level of groundlines GG[0], GG[1] are both set to L level during the non-readingoperation, that is, during waiting or the writing operation. Currently,as either one of N-channel MOS transistors 12, 14 in FIG. 22 has alwaysturned on, the current steadily flows into ground lines GG[0], GG[1].Accordingly, the potential level of both ground lines GG[0], GG[1]attains Vtn.

As described in connection with FIG. 5 of Embodiment 1, since the gateleakage current of the MOS transistor varies exponentially with respectto the gate voltage (generic name of the voltage between the gate andthe source and the voltage between the gate and the drain), slightlowering of the gate voltage results in significant reduction in thegate leakage current. On the other hand, raise of the potential level ofground lines GG[0], GG[1] from ground potential GND by a certainpotential is equivalent to lowering of the potential level of powersupply potential VDD by a certain potential, when the raised potentiallevel is considered as the reference.

Therefore, referring to FIG. 21, level control signals CS[0], CS[1] areboth set to L level so as to set the potential level of ground linesGG[0], GG[1] to Vtn. In this manner, the gate leakage current during thenon-reading operation of memory cell array 110F can significantly bereduced. Thus, power consumption during the non-reading operation ofmemory cell array 110F can significantly be reduced.

A reading operation of memory cell array 110F will now be described. Inthe following, an example in which data in memory cell 1F-0 is read willbe described with reference to FIG. 23.

FIG. 23 shows potential change of ground lines GG[0], GG[1] in memorycell array 110F in Embodiment 6.

When level control signals CS[0], CS[1] are set to H level and L levelrespectively, the potential level of ground line GG[0] in the data readcolumn (selected column) is set to ground potential GND, and thepotential level of ground line GG[1] in the data non-read column(non-selected column) is set to Vtn during the reading operation, asshown in FIG. 23.

As described previously, raise of the potential level of ground linesGG[0], GG[1] from ground potential GND by a certain potential isequivalent to lowering of the potential level of power supply potentialVDD by a certain potential, when the raised potential level isconsidered as the reference.

Therefore, for a reason the same as that described in connection withFIG. 8 of Embodiment 1, the potential level of read bit line RBL[0] ofthe data read column (selected column) is significantly lowered duringthe reading operation. On the other hand, the potential level of readbit line RBL[1] of the data non-read column (non-selected column) is notsignificantly lowered during the reading operation.

Accordingly, read bit line RBL[1] in the data non-read column of whichpotential level is not lowered significantly during the readingoperation can return to H level with a small amount of charging currentin a precharge operation.

Thus, by setting level control signals CS[0], CS[1] to H level and Llevel respectively and by setting the potential of ground lines GG[0],GG[1] to GND and Vtn respectively, power consumption during the readingoperation of memory cell array 110F can be reduced.

When the reading operation is completed, level control signal CS[0] isreturned to L level in order to reduce power consumption by the gateleakage current, and N-channel MOS transistor 31F in FIG. 21 that hasdriven the potential level of ground line GG[0] to ground potential GNDis turned off. Accordingly, the potential level of ground line GG[0] isgradually raised to the level of Vtn and stabilized thereat as shown inFIG. 23.

As described above, according to Embodiment 6, the potential of theground line is controlled in response to the level control signal. Thus,power consumption by the charging/discharging current of the bit line orthe like as well as by the gate leakage current of the memory cell inthe non-selected column can be reduced.

It is to be noted that control of the potential of the ground line inresponse to the level control signal as in Embodiment 6 can also beadapted to Embodiments 1 to 4.

EMBODIMENT 7

Though Embodiments 1 to 6 described a memory cell array constituted ofmulti-port memory cells (2-port, for example), Embodiment 7 willdescribe a memory cell array constituted of single-port memory cells.

FIG. 24 is a circuit diagram showing a circuit configuration of a memorycell array 110G and its periphery in Embodiment 7 of the presentinvention.

Referring to FIG. 24, memory cell array 110G in Embodiment 7 includesmemory cells 1G-0, 1G-1, 1G-2, 1G-3 arranged in matrix of rows andcolumns, word lines WL[0], WL[1] arranged in a direction of row, and bitline pairs BL[0], /BL[0] and BL[1], /BL[1] arranged in a direction ofcolumn.

Word line WL[0] is connected in common to memory cells 1G-0, 1G-1, whileword line WL[1] is connected in common to memory cells 1G-2, 1G-3. Bitline pair BL[0], /BL[0] is connected in common to memory cells 1G-0,1G-2, while bit line pair BL[1], /BL[1] is connected in common to memorycells 1G-1, 1G-3.

Memory cell array 110G in Embodiment 7 is connected to power supply linelevel control circuit 20A via power supply lines VM[0], VM[1], andconnected to ground line level control circuit 30F via ground linesGG[0], GG[1]. Power supply line VM[0] and ground line GG[0] areconnected in common to memory cells 1G-0, 1G-2. Power supply line VM[1]and ground line GG[1] are connected in common to memory cells 1G-1,1G-3.

As the circuit configuration and operation of power supply line levelcontrol circuit 20A has been described in connection with FIG. 2 ofEmbodiment 1, description thereof will not be repeated. In addition, asthe circuit configuration and operation of ground line level controlcircuit 30F has been described in connection with FIG. 21 of Embodiment6, description thereof will not be repeated.

A specific circuit configuration of a memory cell 1G representing memorycells 1G-0, 1G-1, 1G-2, 1G-3 will now be described. Here, memory cell 1Gis assumed to include control lines such as a word line or a bit line.

FIG. 25 is a circuit diagram showing a specific circuit configuration ofmemory cell 1G in Embodiment 7 of the present invention.

Memory cell 1G in Embodiment 7 shown in FIG. 25 has a single-port memorycell configuration. Memory cell 1G includes a data storage portion 1000Gand a write/read port 4000G. Data storage portion 1000G includesinverters 2G, 3G.

Data storage portion 1000G in Embodiment 7 is different from datastorage portion 1000A in Embodiment 1 only in that ground line GG ofwhich potential level can be controlled is connected to inverters 2G,3G. In other words, data storage portion 1000G in Embodiment 7 cancontrol the potential level of both power supply line VM and ground lineGG.

Write/read port 4000G includes N-channel MOS transistors 7, 8, word lineWL, and bit line pair BL, /BL. N-channel MOS transistor 7 has the sourceconnected to storage node N1, has the gate connected to word line WL,and has the drain connected to bit line /BL. N-channel MOS transistor 8has the source connected to storage node N2, has the gate connected toword line WL, and has the drain connected to bit line BL.

An operation of memory cell array 110G including memory cells 1G-0,1G-1, 1G-2, 1G-3 with the memory cell configuration as above will now bedescribed. Note that description of portions the same as those inEmbodiment 1 will not be repeated.

Referring to FIG. 24, in memory cell array 110G in Embodiment 7, levelcontrol signals /CS[0], /CS[1] are both set to H level during thenon-reading operation, that is, during waiting or the writing operation(level control signals CS[0], CS[1] are both set to L level).Accordingly, the potential level of power supply lines VM[0], VM[1] isset to VDD−Vtp, and the potential level of ground lines GG[0], GG[1] isset to Vtn.

As a result, the gate leakage current in the non-reading operation ofmemory cell array 110G can significantly be reduced. Thus, powerconsumption during the non-reading operation of memory cell array 110Gcan significantly be reduced.

A reading operation of memory cell array 110G will now be described. Inthe following, an example in which data in memory cell 1G-0 is read willbe described with reference to FIGS. 26 and 27.

FIG. 26 shows potential change of word line WL[0], bit line pair BL[0],/BL[0], and bit line pair BL[1], /BL[1] in memory cell array 110G inEmbodiment 7.

In an example of the memory cell array constituted of single-port memorycells such as memory cell array 110G in Embodiment 7, adifferential-type memory cell array operation, in which data of H levelor L level is read by detecting lowering of the potential of one of bitline pair BL, /BL arranged for each column, is generally found.

When data in memory cell 1G-0 is read in memory cell array 110G ofEmbodiment 7, word line WL[0] is selected by the row address signal andset to H level (power supply potential VDD) as shown in FIG. 26.Accordingly, the potential of bit line BL[0] out of bit line pair BL[0],/BL[0] is lowered as shown in FIG. 26, and desired data is read. Powersupply potential VDD is set to 1.0V, for example.

As shown in FIG. 24, however, word line WL[0] is also connected tomemory cell 1G-1, which is a memory cell in the non-read column and inthe same row. As such, when the data in memory cell 1G-0 is read to bitline pair BL[0], /BL[0], the data in memory cell 1G-1 is simultaneouslyread to bit line pair BL[1], /BL[1].

In memory cell array 110G in Embodiment 7, each data in memory cells1G-0, 1G-1 that has been read simultaneously is input to a not-shownselector circuit. By selecting the data of one of the bit line pairdesignated by the column address signal, desired data is read.

Desirably, the potential level of bit line BL[0] in the data read columnvaries rapidly in order to attain higher speed in the reading operation,as described in Embodiment 1. On the other hand, desirably, thepotential level of bit line BL[1] in data non-read column does not vary,in order to suppress the charging/discharging current to reduce powerconsumption.

In memory cell array 110G in Embodiment 7, during the reading operation,level control signal /CS[0] controlling the potential level of powersupply line VM[0] in the data read column is set to L level, and levelcontrol signal /CS[1] controlling the potential level of power supplyline VM[1] in the data non-read column is set to H level, respectively.In this manner, level control signal CS[0] controlling the potentiallevel of ground line GG[0] in the data read column is set to H level,and level control signal CS[1] controlling the potential level of groundline GG[1] in the data non-read column is set to L level.

FIG. 27 shows potential change of power supply lines VM[0], VM[1] andground lines GG[0], GG[1] in memory cell array 110G in Embodiment 7.

By setting level control signals /CS[0], /CS[1] to L level and H levelrespectively, in the reading operation, the potential level at powersupply line VM[0] in the data read column (selected column) is set topower supply potential VDD, and the potential level at power supply lineVM[1] in the data non-read column (non-selected column) is set toVDD−Vtp (Vtp is the voltage between the gate and the source of aP-channel MOS transistor), as shown in FIG. 27.

Since level control signals CS[0], CS[1] are set to H level and L levelrespectively, during the reading operation, the potential level atground line GG[0] in the data read column (selected column) is set toground potential GND, and the potential level at ground line GG[1] inthe data non-read column (non-selected column) is set to Vtn (Vtn is thevoltage between the gate and the source of an N-channel MOS transistor),as shown in FIG. 27.

As described previously, raise of the potential level of ground linesGG[0], GG[1] from ground potential GND by a certain potential isequivalent to lowering of the potential level of power supply potentialVDD by a certain potential. Here, control of the potential level ofground lines GG[0], GG[1] will particularly be described in detail.

During the reading operation, the potential level of ground line GG inFIG. 25 in selected memory cell 1G-0 is set to GND, and a voltage ofpower supply potential VDD is applied between the gate and the source ofone of N-channel MOS transistors 12, 14 in FIG. 25. Generally, thehigher the voltage between the gate and the source is, the higher thedrivability of an MOS transistor will be. Therefore, data of bit lineBL[0] in the data read column is rapidly pulled.

Consequently, referring to FIG. 26, the potential level of bit lineBL[0] in the data read column (selected column) is lowered significantlyduring the reading operation.

On the other hand, the potential level of ground line GG in FIG. 25 innon-selected memory cell 1G-1 is set to Vtn, and a voltage of powersupply potential VDD-Vtn is applied between the gate and the source ofone of N-channel MOS transistors 12, 14 in FIG. 25. As non-selectedmemory cell 1G-1 has the voltage between the gate and the source of oneof N-channel MOS transistors 12, 14 lower than that of selected memorycell 1G-0, the data of bit line BL[1] in the data non-read column ispulled slowly.

Consequently, referring to FIG. 26, the potential level of bit lineBL[1] in the data non-read column (non-selected column) is not loweredsignificantly during the reading operation.

When the potential change of bit line BL[0] is transmitted to dataoutput and the reading operation is completed, the potential level ofbit lines BL[0], BL[1] return to H level by precharging, as shown inFIG. 26.

Here, bit line BL[1] in the data non-read column of which potentiallevel is not lowered significantly during the reading operation canreturn to H level with a small amount of charging current in theprecharge operation.

In addition, as the voltage between the gate and the source of the MOStransistor that has turned on in non-selected memory cell 1G-1 is lowerthan power supply potential VDD, the leakage current in that MOStransistor can be reduced.

Meanwhile, the potential level of ground line GG in FIG. 25 innon-selected memory cell 1G-1 is set to Vtn, that is, floats from groundpotential GND. Therefore, when the substrate potential of the N-channelMOS transistor in non-selected memory cell 1G-1 is set to groundpotential GND, reverse bias of the Vtn potential is applied between thesubstrate and the source of the N-channel MOS transistor.

Consequently, the threshold voltage of the N-channel MOS transistor innon-selected memory cell 1G-1 is raised, and the leakage current betweenthe source and the drain of the N-channel MOS transistor can be reduced.

Similarly, as the potential level of power supply line VM in FIG. 25 innon-selected memory cell 1G-1 is set to VDD−Vtp, reverse bias of the Vtppotential is applied between the substrate and the source of theP-channel MOS transistor in non-selected memory cell 1G-1

Consequently, the threshold voltage of the P-channel MOS transistor innon-selected memory cell 1G-1 is raised, and the leakage current betweenthe source and the drain of the P-channel MOS transistor can be reduced.

As described above, according to Embodiment 7, the potential of thepower supply line and the ground line is controlled in response to thelevel control signal. Thus, power consumption by thecharging/discharging current of the bit line or the like as well as bythe gate leakage current of the memory cell in the non-selected columncan be reduced.

Though Embodiment 7 has described an example in which both power supplyline VM and ground line GG are controlled by a unit of columns withrespect to the memory cell array having a single-port memory cellconfiguration, it is possible to control only one of power supply lineVM and ground line GG.

EMBODIMENT 8

Though Embodiments 1 to 7 have described a memory cell array and itsperipheral circuit, or a memory cell array, Embodiment 8 will describeone example of the power supply line level control circuit inEmbodiments 1, 2, 5, and 7.

FIG. 28 is a circuit diagram showing a circuit configuration of powersupply line level control circuit 20 in Embodiment 8 of the presentinvention.

Referring to FIG. 28, power supply line level control circuit 20 inEmbodiment 8 includes power supply line level switching circuits 200-0,200-1 provided for each column. Power supply line level switchingcircuit 200-0 controls the potential level of power supply line VM[0],in response to a retention test control signal RT, a redundancyreplacement control signal KILL[0], retention potential setting signalsDCL0, DCL1, DCL2, and level control signal CS[0]. Power supply linelevel switching circuit 200-1 controls the potential level of powersupply line VM[1], in response to retention test control signal RT, aredundancy replacement control signal KILL[1], retention potentialsetting signals DCL0, DCL1, DCL2, and level control signal CS[1]. Aspecific circuit configuration of power supply line level switchingcircuit 200 representing power supply line level switching circuits200-0, 200-1 will now be described.

FIG. 29 is a circuit diagram showing a specific circuit configuration ofpower supply line level switching circuit 200 in Embodiment 8 of thepresent invention.

Referring to FIG. 29, power supply line level switching circuit 200 ofEmbodiment 8 includes an NAND circuit 201 receiving level control signalCS and redundancy replacement control signal KILL, P-channel MOStransistors 202 to 204, 206, 209 having the drains connected to powersupply line VM, and diode-connected P-channel MOS transistors 205, 207,208.

P-channel MOS transistors 202, 203, 209 have the sources connected topower supply node VDD. P-channel MOS transistor 202 receives an outputof NAND circuit 201 at its gate. P-channel MOS transistor 203 receivesretention potential setting signal DCL0 at its gate. P-channel MOStransistor 209 receives retention test control signal RT at its gate.

P-channel MOS transistors 204, 205 are connected in series between powersupply node VDD and power supply line VM. P-channel MOS transistor 204receives retention potential setting signal DCL1 at its gate. P-channelMOS transistors 206, 207, 208 are connected in series between powersupply node VDD and power supply line VM. P-channel MOS transistor 206receives retention potential setting signal DCL2 at its gate.

FIG. 30 illustrates an operation of power supply line level switchingcircuit 200 in Embodiment 8 of the present invention.

Initially, an example in which level control signal CS also serving asthe column select signal is at H level, that is, an example in which acolumn is selected for access, will be described. Here, redundancyreplacement control signal KILL is set to H level. This means that theselected column does not contain a defective cell and a normal operationis attained. Therefore, this column is not replaced with a spare column,and is actually accessed. Here, retention test control signal RT andretention potential setting signals DCL0, DCL1, DCL2 may be at H levelor L level during access. FIG. 30 shows with “X” a state where any of Hlevel and L level may be accepted.

Referring to FIG. 29, when level control signal CS and redundancyreplacement control signal KILL are both at H level, NAND circuit 201outputs a signal of L level. In response to this, P-channel MOStransistor 202 turns on, and the potential level of power supply line VMattains power supply potential VDD, as also shown in FIG. 30.

Referring back to FIG. 30, an example in which level control signal CSalso serving as the column select signal is at L level, that is,non-access in which a column is neither selected nor accessed, will bedescribed. Here, retention test control signal RT is set to H level.Moreover, one of retention potential setting signals DCL0, DCL1, DCL2 isset to L level, and remaining two signals are set to H level. Here,redundancy replacement control signal KILL may be at H level or L level.

Referring to FIG. 29, when level control signal CS is at L level, NANDcircuit 201 outputs a signal of L level regardless of H level/L level ofredundancy replacement control signal KILL. In response to this,P-channel MOS transistor 202 turns off. As retention test control signalRT is also at H level, P-channel MOS transistor 209 also turns off.

In non-access, when retention potential setting signal DCL2 is at Llevel and retention potential setting signals DCL0, DCL1 are at H level,only P-channel MOS transistor 206 out of P-channel MOS transistors 203,204, 206 turns on, and P-channel MOS transistors 203, 204 turn off.Accordingly, the potential level of power supply line VM is set toVDD−2Vtp (Vtp is the voltage between the gate and the source of aP-channel MOS transistor) as also shown in FIG. 30.

In non-access, when retention potential setting signal DCL1 is at Llevel and retention potential setting signals DCL0, DCL2 are at H level,only P-channel MOS transistor 204 out of P-channel MOS transistors 203,204, 206 turns on, and P-channel MOS transistors 203, 206 turn off.Accordingly, the potential level of power supply line VM is set toVDD−Vtp as also shown in FIG. 30.

In non-access, when retention potential setting signal DCL0 is at Llevel and retention potential setting signals DCL1, DCL2 are at H level,only P-channel MOS transistor 203 out of P-channel MOS transistors 203,204, 206 turns on, and P-channel MOS transistors 204, 206 turn off.Accordingly, the potential level of power supply line VM is set to powersupply potential VDD as also shown in FIG. 30.

In this manner, power supply line level switching circuit 200 can switchthe potential level of power supply line VM during non-access bychanging a combination of H level/L level of retention potential settingsignals DCL0, DCL1, DCL2. By allowing for switch between the potentiallevel of power supply line VM, variation in a value of power supplypotential VDD can be addressed in a flexible manner.

For example, when power supply potential VDD is lower than a prescribedvalue, and if the potential level of power supply line VM is lowered toVDD−2Vtp during non-access, the potential level of power supply line VMbecomes too low, and data in the memory cell connected to power supplyline VM may not be held correctly. In such a case, by switching thepotential level of power supply line VM to VDD−Vtp for example, the datain the memory cell can correctly be held. When the potential level ofpower supply line VM is too low even after switching the potential levelof power supply line VM to VDD−Vtp, the potential level of power supplyline VM should only be switched to power supply potential VDD.

Referring back to FIG. 30, an example of redundancy replacement will bedescribed. Here, redundancy replacement control signal KILL is set to Llevel. This means that the selected column contains a defective cell andthe normal operation is not attained. Therefore, this column is replacedwith a spare column, and is not actually accessed. Here, retention testcontrol signal RT and retention potential setting signals DCL0, DCL1,DCL2 are all set to H level. Note that level control signal CS may be atH level or L level in redundancy selection.

Referring to FIG. 29, when redundancy replacement control signal KILL isat L level, NAND circuit 201 outputs a signal of L level regardless of Hlevel/L level of level control signal CS. In response to this, P-channelMOS transistor 202 turns off. As retention test control signal RT andretention potential setting signals DCL0, DCL1, DCL2 are all at H level,all of P-channel MOS transistors 203, 204, 206, 209 also turn off. As aresult, power supply line VM is set to floating with high impedance(Hi-Z), as also shown in FIG. 30.

Though the selected column is not actually accessed in redundancyselection, the leakage current in that column may abnormally becomeslarge if the column contains a defect such as short-circuit. By settingpower supply line VM to floating in redundancy selection, such anabnormal leakage current can be suppressed.

Referring back to FIG. 30, an example of retention test will bedescribed. Retention test represents a mode to test a data hold propertyof a memory cell, and is not employed in an ordinary operation state.Here, retention test control signal RT and level control signal CS areboth set to L level. Retention potential setting signals DCL0, DCL1,DCL2 are all set to H level. Note that redundancy replacement controlsignal KILL may be at H level or L level in the retention test.

Referring to FIG. 29, when level control signal CS is at L level, NANDcircuit 201 outputs a signal of L level regardless of H level/L level ofredundancy replacement control signal KILL. In response to this,P-channel MOS transistor 202 turns off. As retention potential settingsignals DCL0, DCL1, DCL2 are all at H level, all of P-channel MOStransistors 203, 204, 206 turn off.

On the other hand, as retention test control signal RT is set to Llevel, P-channel MOS transistor 209 turns on. Drivability of P-channelMOS transistor 209 is set to be sufficiently small. Meanwhile,drivability required for the memory cell to hold the storage datacorrectly when the memory cell connected to power supply line VM doesnot contain a defective memory cell is maintained.

Consider an example in which a memory cell with a large leakage currentexists among memory cells connected to power supply line VM. In anordinary operation test, when a memory cell is accessed, P-channel MOStransistor 202 turns on. Accordingly, even if a memory cell with a largeleakage current exists, reading/writing of the memory cell is normallycompleted due to the drivability of P-channel MOS transistor 202.Therefore, in some cases, the conventional, ordinary operation testcould not determine the memory cell with a large leakage current torepair the same by redundancy replacement.

In the retention test of the present invention, P-channel MOS transistor202 is turned off, and drivability of P-channel MOS transistor 209 thatturns on is set to be sufficiently small. Therefore, if a memory cellwith a large leakage current exists, the potential level of power supplyline VM is lowered by an influence of that memory cell. As such, thememory cell connected to power supply line VM cannot hold the datacorrectly, and the result of retention test turns out to be defective(fail). By replacing the defective memory cell with a spare column basedon this result indicating defect, abnormal leakage current can beeliminated.

As described above, according to Embodiment 8, setting of a variety ofcontrol signals input to the power supply line level control circuit ischanged in accordance with an operation mode of the memory cell array,whereby optimal setting of the potential level of the power supply linefor each operation mode of the memory cell array can be achieved.

EMBODIMENT 9

Embodiment 9 describes one example of the ground line level controlcircuit in Embodiments 5, 6 and 7.

FIG. 31 is a circuit diagram showing a circuit configuration of groundline level control circuit 30 in Embodiment 9 of the present invention.

Referring to FIG. 31, ground line level control circuit 30 in Embodiment9 includes ground line level switching circuits 300-0, 300-1 providedfor each column. Ground line level switching circuit 300-0 controls thepotential level of ground line GG[0], in response to a retention testcontrol signal /RT, a redundancy replacement control signal /KILL[0],retention potential setting signals /DCL0, /DCL1, /DCL2, and levelcontrol signal /CS[0]. Ground line level switching circuit 300-1controls the potential level of ground line GG[1], in response toretention test control signal /RT, a redundancy replacement controlsignal /KILL[1], retention potential setting signals /DCL0, /DCL1,/DCL2, and level control signal /CS[1]. A specific circuit configurationof ground line level switching circuit 300 representing ground linelevel switching circuits 300-0, 300-1 will now be described.

FIG. 32 is a circuit diagram showing a specific circuit configuration ofground line level switching circuit 300 in Embodiment 9 of the presentinvention.

Referring to FIG. 32, ground line level switching circuit 300 ofEmbodiment 9 includes an NOR circuit 301 receiving level control signal/CS and redundancy replacement control signal /KILL, N-channel MOStransistors 302 to 304, 306, 309 having the drain connected to groundline GG, and diode-connected N-channel MOS transistors 305, 307, 308.

N-channel MOS transistors 302, 303, 309 have the sources connected toground node GND. N-channel MOS transistor 302 receives an output of NORcircuit 301 at its gate. N-channel MOS transistor 303 receives retentionpotential setting signal /DCL0 at its gate. N-channel MOS transistor 309receives retention test control signal /RT at its gate.

N-channel MOS transistors 304, 305 are connected in series betweenground node GND and ground line GG. N-channel MOS transistor 304receives retention potential setting signal /DCL1 at its gate. N-channelMOS transistors 306, 307, 308 are connected in series between groundnode GND and ground line GG. N-channel MOS transistor 306 receivesretention potential setting signal /DCL2 at its gate.

FIG. 33 illustrates an operation of ground line level switching circuit300 in Embodiment 9 of the present invention.

Initially, an example in which level control signal /CS also serving asthe column select signal is at L level, that is, an example in which acolumn is selected for access, will be described. Here, redundancyreplacement control signal /KILL is set to L level. This means that theselected column does not contain a defective cell and the normaloperation is attained. Therefore, this column is not replaced with aspare column, and is actually accessed. Here, retention test controlsignal /RT and retention potential setting signals /DCL0, /DCL1, /DCL2may be at H level or L level during access. FIG. 33 shows with “X” astate where any of H level and L level may be accepted.

Referring to FIG. 32, when level control signal /CS and redundancyreplacement control signal /KILL are both at H level, NOR circuit 301outputs a signal of H level. In response to this, N-channel MOStransistor 302 turns on, and the potential level of ground line GGattains ground potential GND, as also shown in FIG. 33.

Referring back to FIG. 33, an example in which level control signal /CSalso serving as the column select signal is at L level, that is,non-access in which a column is neither selected nor accessed, will bedescribed. Here, retention test control signal /RT is set to L level.Moreover, one of retention potential setting signals /DCL0, /DCL1, /DCL2is set to H level, and remaining two signals are set to L level. Here,redundancy replacement control signal /KILL may be at H level or Llevel.

Referring to FIG. 32, when level control signal /CS is at H level, NORcircuit 301 outputs a signal of L level regardless of H level/L level ofredundancy replacement control signal /KILL. In response to this,N-channel MOS transistor 302 turns off. As retention test control signal/RT is also at L level, N-channel MOS transistor 309 also turns off.

In non-access, when retention potential setting signal /DCL2 is at Hlevel and retention potential setting signals /DCL0, /DCL1 are at Llevel, only N-channel MOS transistor 306 out of N-channel MOStransistors 303, 304, 306 turns on, and N-channel MOS transistors 303,304 turn off. Accordingly, the potential level of ground line GG is setto GND+2Vtn (Vtn is the voltage between the gate and the source of anN-channel MOS transistor) as also shown in FIG. 33.

In non-access, when retention potential setting signal /DCL1 is at Hlevel and retention potential setting signals /DCL0, /DCL2 are at Llevel, only N-channel MOS transistor 304 out of N-channel MOStransistors 303, 304, 306 turns on, and N-channel MOS transistors 303,306 turn off. Accordingly, the potential level of ground line GG is setto GND+Vtn as also shown in FIG. 33.

In non-access, when retention potential setting signal /DCL0 is at Hlevel and retention potential setting signals /DCL1, /DCL2 are at Llevel, only N-channel MOS transistor 303 out of N-channel MOStransistors 303, 304, 306 turns on, and N-channel MOS transistors 304,306 turn off. Accordingly, the potential level of ground line GG is setto ground potential GND as also shown in FIG. 33.

In this manner, ground line level switching circuit 300 can switch thepotential level of ground line GG during non-access by changing acombination of H level/L level of retention potential setting signals/DCL0, /DCL1, /DCL2. By allowing for switch between the potential levelof ground line GG, variation in a value of ground potential GND can beaddressed in a flexible manner.

For example, when ground potential GND is higher than a prescribedvalue, and if the potential level of ground line GG is raised toGND+2Vtn during non-access, the potential level of ground line GGbecomes too high, and data in the memory cell connected to ground lineGG may not be held correctly. In such a case, by switching the potentiallevel of ground line GG to GND+Vtn for example, the data in the memorycell can correctly be held. When the potential level of ground line GGis too high even after switching the potential level of ground line GGto GND+Vtn, the potential level of ground line GG should only beswitched to ground potential GND.

Referring back to FIG. 33, an example of redundancy replacement will bedescribed. Here, redundancy replacement control signal /KILL is set to Hlevel. This means that the selected column contains a defective cell andthe normal operation is not attained. Therefore, this column is replacedwith a spare column, and is not actually accessed. Retention testcontrol signal /RT and retention potential setting signals IDCL0, /DCL1,/DCL2 are all set to L level. Here, level control signal /CS may be at Hlevel or L level in redundancy selection.

Referring to FIG. 32, when redundancy replacement control signal /KILLis at H level, NOR circuit 301 outputs a signal of L level regardless ofH level/L level of level control signal /CS. In response to this,N-channel MOS transistor 302 turns off. As retention test control signal/RT and retention potential setting signals /DCL0, /DCL1, /DCL2 are allat L level, all of N-channel MOS transistors 303, 304, 306, 309 alsoturn off. As a result, ground line GG is set to floating with highimpedance (Hi-Z), as also shown in FIG. 33.

Though the selected column is not actually accessed in redundancyselection, the leakage current in that column may abnormally becomeslarge if the column contains a defect such as short-circuit. By settingground line GG to floating in redundancy selection, such an abnormalleakage current can be suppressed.

Referring back to FIG. 33, an example of retention test will bedescribed. Retention test represents a mode to test a data hold propertyof a memory cell, and is not employed in an ordinary operation state.Here, retention test control signal /RT and level control signal /CS areboth set to H level. Retention potential setting signals /DCL0, /DCL1,/DCL2 are all at L level. Redundancy replacement control signal /KILLmay be at H level or L level in the retention test.

Referring to FIG. 32, when level control signal /CS is at H level, NORcircuit 301 outputs a signal of L level regardless of H level/L level ofredundancy replacement control signal /KILL. In response to this,N-channel MOS transistor 302 turns off. As retention potential settingsignals /DCL0, /DCL1, /DCL2 are all at L level, all of N-channel MOStransistors 303, 304, 306 turn off.

On the other hand, as retention test control signal /RT is at H level,N-channel MOS transistor 309 turns on. Drivability of N-channel MOStransistor 309 is set to be sufficiently small. Meanwhile, drivabilityrequired for the memory cell to hold the storage data correctly when thememory cell connected to ground line GG does not contain a defectivememory cell is maintained.

Consider an example in which a memory cell with a large leakage currentexists among memory cells connected to ground line GG. In an ordinaryoperation test, when a memory cell is accessed, N-channel MOS transistor302 turns on. Accordingly, even if a memory cell with a large leakagecurrent exists, reading/writing of the memory cell is normally completeddue to drivability of N-channel MOS transistor 302. Therefore, theconventional, ordinary operation test could not determine the memorycell with a large leakage current to repair the same by redundancyreplacement.

In the retention test of the present invention, N-channel MOS transistor302 is turned off, and drivability of N-channel MOS transistor 309 whichturns on is set to be sufficiently small. Therefore, if a memory cellwith a large leakage current exists, the potential level of ground lineGG is lowered by an influence of that memory cell. As such, the memorycell connected to ground line GG cannot hold the data correctly, and theresult of retention test turns out to be defective (fail). By replacingthe defective memory cell with a spare column based on this resultindicating defect, abnormal leakage current can be eliminated.

As described above, according to Embodiment 9, setting of a variety ofcontrol signals input to the ground line level control circuit ischanged in accordance with an operation mode of the memory cell array,whereby optimal setting of the potential level of the ground line foreach operation mode of the memory cell array can be achieved.

EMBODIMENT 10

Embodiment 10 describes a setting signal control circuit controlling thelogic level of retention potential setting signals DCL0, DCL1, DCL2described in Embodiments 8 and 9 in accordance with magnitude of powersupply potential VCC.

FIG. 34 is a circuit diagram showing a circuit configuration of asetting signal control circuit 500 in Embodiment 10 of the presentinvention.

Referring to FIG. 34, setting signal control circuit 500 in Embodiment10 includes potential level adjustment circuits 510 a, 510 b, transfergates 520 a, 520 b, latch circuits 530 a, 530 b, NAND circuits 541, 543,545, and inverters 542, 544.

Potential level adjustment circuit 510 a includes a diode-connectedP-channel MOS transistor 511, and a P-channel MOS transistor 512 and anN-channel MOS transistor 513 connected to a node N11. P-channel MOStransistors 511, 512 and N-channel MOS transistor 513 are connected inseries between power supply node VDD and ground node GND. P-channel MOStransistor 512 and N-channel MOS transistor 513 receive a clock signalCLK at their gates.

In potential level adjustment circuit 510 a, when clock signal CLK is atL level, P-channel MOS transistor 512 turns on, and N-channel MOStransistor 513 turns off. As a result, the potential level of node N11is set to VDD−Vtp (Vtp is the voltage between the gate and the source ofa P-channel MOS transistor). On the other hand, when clock signal CLK isat H level, P-channel MOS transistor 512 turns off, and N-channel MOStransistor 513 turns on. As a result, the potential level of node N11 isset to ground potential GND.

Potential level adjustment circuit 510 b includes diode-connectedP-channel MOS transistors 514, 515, and a P-channel MOS transistor 516and an N-channel MOS transistor 517 connected to node N11. P-channel MOStransistors 514, 515, 516 and N-channel MOS transistor 517 are connectedin series between power supply node VDD and ground node GND. P-channelMOS transistor 516 and N-channel MOS transistor 517 receive clock signalCLK at their gates.

In potential level adjustment circuit 510 b, when clock signal CLK is atL level, P-channel MOS transistor 516 turns on, and N-channel MOStransistor 517 turns off. As a result, the potential level of a node N21is set to VDD−2Vtp. On the other hand, when clock signal CLK is at Hlevel, P-channel MOS transistor 516 turns off, and N-channel MOStransistor 517 turns on. As a result, the potential level of node N21 isset to ground potential GND.

Transfer gate 520 a is connected to node N11. Transfer gate 520 a passesan input signal at potential level VDD−Vtp input from potential leveladjustment circuit 510 a when clock signal CLK is at L level. On theother hand, transfer gate 520 a isolates an input signal at groundpotential GND input from potential level adjustment circuit 510 a whenclock signal CLK is at H level.

Transfer gate 520 b is connected to node N21. Transfer gate 520 b passesan input signal at potential level VDD−2Vtp input from potential leveladjustment circuit 510 b when clock signal CLK is at L level. On theother hand, transfer gate 520 b isolates an input signal at groundpotential GND input from potential level adjustment circuit 510 b whenclock signal CLK is at H level.

Latch circuit 530 a includes inverters 531 a, 532 a connected so as toform a ring. Latch circuit 530 a provides an output signal to a node N12in response to an input signal at potential level VDD−Vtp input frompotential level adjustment circuit 510 a via transfer gate 520 a whenclock signal CLK is at L level. Latch circuit 530 a inverts the inputsignal when potential level VDD−Vtp thereof is higher than an inputthreshold voltage Vth. On the other hand, latch circuit 530 a enters adata hold state because transfer gate 520 a is isolated when clocksignal CLK is at L level.

Latch circuit 530 b includes inverters 531 b, 532 b connected so as toform a ring. Latch circuit 530 b provides an output signal to a node N22in response to an input signal at potential level VDD−2Vtp input frompotential level adjustment circuit 510 b via transfer gate 520 b whenclock signal CLK is at L level. Latch circuit 530 b inverts the inputsignal when potential level VDD−2Vtp thereof is higher than inputthreshold voltage Vth. On the other hand, latch circuit 530 b enters adata hold state because transfer gate 520 b is isolated when clocksignal CLK is at L level.

NAND circuit 541 outputs retention potential setting signal DCL0 inresponse to signals from nodes N12, N22. Inverter 542 inverts a signalfrom node N12. NAND circuit 543 outputs retention potential settingsignal DCL1 in response to signals from inverter 542 and node N22.Inverter 544 inverts a signal from node N22. NAND circuit 545 outputsretention potential setting signal DCL2 in response to signals frominverters 542, 544.

FIGS. 35A to 35D are operational waveform diagrams illustrating anoperation of setting signal control circuit 500 respectively inEmbodiment 10 of the present invention.

FIG. 35A shows change in the potential level of clock signal CLK. Asshown in FIG. 35A, clock signal CLK rises to H level (power supplypotential VDD) at time t1 and t3, and falls to L level at time t2 andt4.

FIGS. 35B, 35C and 35D all show change in the potential level at nodesN11 and N21. As described in connection with FIG. 34, time t1 to t4 atwhich the potential level of nodes N11, N21 changes in FIGS. 35B, 35C,35D is in synchronization with time t1 to t4 at which the potentiallevel of clock signal CLK changes in FIG. 35A.

As shown in FIGS. 35B, 35C and 35D, the potential level at node N11falls at time t1 and t3, and rises to VDD−Vtp at time t2, t4. Inaddition, the potential level at node N21 falls at time t1 and t3, andrises to VDD−2Vtp at time t2, t4.

In this manner, the potential level of nodes N11, N21 are set to VDD−Vtpand VDD−2Vtp respectively when clock signal CLK is at L level. On theother hand, these potential levels VDD−Vtp and VDD−2Vtp vary inaccordance with variation in the value of power supply potential VDD.FIGS. 35B, 35C and 35D illustrate the potential level of nodes N11, N21when power supply potential VDD is varied in three examples in thefollowing, based on a relation with input threshold voltage Vth in latchcircuits 530 a, 530 b in FIG. 34.

FIG. 35B shows change in the potential level of nodes N11, N21 whenpotential levels VDD−Vtp and VDD−2Vtp are both higher than inputthreshold voltage Vth.

As shown in FIG. 35B, when power supply potential VDD is sufficientlyhigh, potential levels VDD−Vtp and VDD−2Vtp of the input signalsprovided to latch circuits 530 a, 530 b in FIG. 34 are both higher thaninput threshold voltage Vth. Here, as described in connection with FIG.34, latch circuits 530 a, 530 b in FIG. 34 both invert the input signalwhen clock signal CLK is at L level. As a result, the potential level ofnodes N12, N22 are both set to L level.

In response to this, retention potential setting signals DCL0, DCL1,DCL2 are set to H level, H level and L level respectively, referring toFIG. 34. These retention potential setting signals DCL0, DCL1, DCL2 areinput to power supply line level switching circuit 200 in FIG. 29, andretention test control signal RT and level control signal CS are set toH level and L level respectively, whereby the potential level of powersupply line VM is set to VDD−2Vtp, as shown in FIG. 30.

As described above, when power supply potential VDD is sufficientlyhigh, retention potential setting signals DCL0, DCL1, DCL2 are set to Hlevel, H level and L level respectively. Consequently, the potentiallevel of power supply line VM is set to as low as VDD−2Vtp.

FIG. 35C shows change in the potential level of nodes N11, N21 whenpotential level VDD−Vtp is higher than input threshold voltage Vth andpotential level VDD−2Vtp is lower than input threshold voltage Vth.

As shown in FIG. 35C, when power supply potential VDD is lower than inthe example of FIG. 35B, potential level VDD−Vtp of the input signalprovided to latch circuit 530 a in FIG. 34 is higher than inputthreshold voltage Vth, and potential level VDD−2Vtp of the input signalprovided to latch circuit 530 b in FIG. 34 is lower than input thresholdvoltage Vth.

Here, as described in connection with FIG. 34, when clock signal CLK isat L level, latch circuit 530 a in FIG. 34 inverts the input signal,whereas latch circuit 530 b in FIG. 34 does not invert the same. As aresult, the potential level of nodes N12, N22 are set to L level and Hlevel respectively.

In response to this, retention potential setting signals DCL0, DCL1,DCL2 are set to H level, L level and H level respectively, referring toFIG. 34. These retention potential setting signals DCL0, DCL1, DCL2 areinput to power supply line level switching circuit 200 in FIG. 29, andretention test control signal RT and level control signal CS are set toH level and L level respectively, whereby the potential level of powersupply line VM is set to VDD−Vtp, as shown in FIG. 30.

As described above, when power supply potential VDD is lower than in theexample of FIG. 35B, retention potential setting signals DCL0, DCL1,DCL2 are set to H level, L level and H level respectively. As a result,the potential level of power supply line VM is set to VDD−Vtp, andlowering of the potential level can be suppressed.

FIG. 35D shows change in the potential level of nodes N11, N21 when thepotential levels VDD−Vtp and VDD−2Vtp are both lower than inputthreshold voltage Vth.

As shown in FIG. 35D, when power supply potential VDD is further lowerthan in the example of FIG. 35C, potential levels VDD−Vtp and VDD−2Vtpof the input signal provided to latch circuits 530 a, 530 b in FIG. 34are both lower than input threshold voltage Vth. Here, as described inconnection with FIG. 34, neither of latch circuits 530 a, 530 b in FIG.34 inverts the input signal when clock signal CLK is at L level. As aresult, the potential level of nodes N12, N22 are both set to H level.

In response to this, retention potential setting signals DCL0, DCL1,DCL2 are set to L level, H level and H level respectively, referring toFIG. 34. These retention potential setting signals DCL0, DCL1, DCL2 areinput to power supply line level switching circuit 200 in FIG. 29, andretention test control signal RT and level control signal CS are set toH level and L level respectively, whereby the potential level of powersupply line VM is set to power supply potential VDD, as shown in FIG.30.

As described above, when power supply potential VDD is further lowerthan in the example of FIG. 35C, retention potential setting signalsDCL0, DCL1, DCL2 are set to L level, H level and H level respectively.Consequently, the potential level of power supply line VM can bereturned to power supply potential VDD.

Therefore, setting signal control circuit 500 in Embodiment 10 cancontrol each logic level of retention potential setting signals DCL0,DCL1, DCL2 in accordance with variation in power supply potential VDD,if it happens. As such, even if power supply potential VDD varies, thepotential level of power supply line VM can automatically be adjusted toan optimal value, so as not to deteriorate the data hold property of thememory cell during non-access due to variation in power supply potentialVDD.

In chip designing in a semiconductor memory device of recent days, inorder to achieve lower power consumption, the power supply voltage isdynamically fluctuated in accordance with an operation status. Settingsignal control circuit 500 in Embodiment 10 can adapt to suchfluctuation of the power supply voltage.

Note that lowering of the potential level of power supply potential VDDby a certain value is equivalent to raise of ground potential GND bythat value, when the lowered potential level is considered as reference.Therefore, setting signal control circuit 500 can supply retentionpotential setting signals /DCL0, /DCL1, /DCL2 to ground line levelcontrol circuit 30 in Embodiment 9.

As described above, according to Embodiment 10, the logic level ofretention potential setting signals DCL0, DCL1, DCL2 is controlled inaccordance with variation in power supply potential VDD. Therefore, evenif power supply potential VDD varies, the potential level of powersupply line VM can automatically be adjusted to an optimal value.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1-18. (canceled)
 19. A semiconductor memory device, comprising: aplurality of memory cells arranged in matrix of rows and columns; aplurality of write word lines arranged individually for each of saidplurality of memory cells, each of said plurality of memory cellsincluding a data storage portion holding data, a data write portionwriting data into said data storage portion, and a data read portionhaving a read bit line for reading data from said data storage portion,and said data storage portion having first and second inverter circuitsconnected in common to a ground line arranged corresponding torespective columns of said plurality of memory cells; and a ground linelevel control circuit controlling a potential level of said ground lineto a ground potential or to a prescribed potential level higher than theground potential in response to a level control signal set for eachcolumn.
 20. The semiconductor memory device according to claim 19, saidground line level control circuit controls the potential level of saidground line to said prescribed potential level for each column during anon-reading operation, and controls the potential level of said groundline to the ground potential with respect to a selected column, and tosaid prescribed potential level with respect to a non-selected columnrespectively during a reading operation.
 21. The semiconductor memorydevice according to claim 19, further comprising a second ground linelevel control circuit controlling the potential level of a ground lineto the ground potential, a power supply potential or floating for eachcolumn in response to said level control signal, wherein said data readportion includes a transistor having a gate connected to a read terminalof said data storage portion, and having a source connected to theground line of which potential level can be controlled.
 22. Thesemiconductor memory device according to claim 21, wherein said secondground line level control circuit controls the potential level of saidground line to said prescribed potential level for each column during anon-reading operation, and controls the potential level of said groundline to the ground potential with respect to a selected column, and tothe power supply potential or floating with respect to a non-selectedcolumn respectively during a reading operation.
 23. The semiconductormemory device according to claim 19, wherein said ground line levelcontrol circuit includes a plurality of ground line level switchingcircuits provided for each column and switching the potential level ofsaid ground line to the ground potential, a plurality of prescribedpotential levels higher than the ground potential, or floating for eachcolumn, in response to at least one of a retention test control signal,a redundancy replacement control signal, a plurality of retentionpotential setting signals, and said level control signal.
 24. Thesemiconductor memory device according to claim 23, said ground linelevel switching circuit switches the potential level of said ground lineto the ground potential during access and retention test, switches thepotential level of said ground line to the ground potential or saidplurality of prescribed potential levels during non-access, and switchesthe potential level of said ground line to floating during redundancyreplacement.
 25. The semiconductor memory device according to claim 23,further comprising a setting signal control circuit controlling a logiclevel of said plurality of retention potential setting signals insynchronization with a clock signal in accordance with magnitude of thepower supply potential.
 26. The semiconductor memory device according toclaim 25, wherein said setting signal control circuit includes a firstpotential level adjustment circuit outputting a first power supplypotential shift signal having the potential level lower than the powersupply potential by a prescribed value, in synchronization with saidclock signal, a second potential level adjustment circuit outputting asecond power supply potential shift signal having the potential levellower than said first potential level by a prescribed value, insynchronization with said clock signal, a first latch circuit outputtinga first select control signal in accordance with a result obtained bycomparison with an input threshold voltage, upon receiving said firstpower supply potential shift signal, and a second latch circuitoutputting a second select control signal in accordance with a resultobtained by comparison with said input threshold voltage, upon receivingsaid second power supply potential shift signal, and said setting signalcontrol circuit controls the logic level of said plurality of retentionpotential setting signals in accordance with a combination of said firstand second select control signals. 27-38. (canceled)